As much as paper weight matters, what else does matter is the coating (aka the finish) that goes onto the card as part of the process for it.
This is a common misconception. The COATING is not the same as the FINISH.
Finish is a card's texture. That texture can be imparted by a variety of methods. USPC and many others today typically use a pair of metal rollers that will impress a finish into the surface of the cards as the two layers of paper are glued together to create the pasteboard on which quality modern playing cards are usually made. Sometimes that texture can be created by adding a gloss layer of some kind at varying depths - the MPC Impressions series of playing cards is a prime example, creating a three-dimensional feel to the surface of the card through the application of layers of "UV ink." In the old days (1970s and earlier), playing cards used to get their texture by having a coating applied to the paper with a fabric roller, which creates a textured surface in a similar way to how a paint roller applies paint to a wall. But whereas the paint is supposed to dry flat, the coating retained the texture of the cloth - and it's why many finishes to this day are named after various types of fabric, such as cambric and linen.
It's because of this old-school method of applying texture that people are often confused about what is a card's finish - it's the texture, not the coating. USPC doesn't help matters by naming their newest high-end coating "Magic Finish," though it was originally called "Performance Coating" while it was still in development, a name which the Ellusionist Playing Card Company still uses to this day when producing decks through USPC. As a magician, I hate the idea of whipping out a deck of cards and someone spotting the word "Magic" anywhere on the box, even if it is "Magic Finish!"
Medicus -
I like the basic idea, but I'd suggest not getting too esoteric with the design concept and stick with making the art as GORGEOUS as humanly possible. It's what will sell your deck.
Faux aging was and still is popular, but as a design concept it's pretty played out. I think that for beautifully artistic decks, it can actually be a detriment, distracting and muddying the artwork. Perhaps consider instead using a look of faux parchment, maybe with an ivory color - something more likely to accentuate the artwork and really show it off instead of making it appear as if it's been sitting in a trunk getting dusty for decades.
It's a design concept I like to compare a lot to Western movies made in the modern day. When you think of a Western, you think of dust and grit and dirt and things looking weathered and aged a hundred years or more. But when the Old West was just the West, in the post-Civil War years, was everything really all old and worn-out in appearance? Dusty, perhaps, and maybe even dirty - there were a lot of desert towns back then and Windex and Fantastik hadn't been invented yet! But ancient and worn-looking? Not really - they didn't use hundred-year-old lumber that had been sitting in the rain when they built their towns, did they? They used new lumber, maybe some paint or stain - things had a new, raw look to them, perhaps, not all ancient and weathered. We think of them as old as weathered because that's what the Old West looks like TODAY, and that's what Hollywood has stylized the Old West to look like in so many movies that if you really made the Old West look they way it did when it was new, many audiences might not even recognize it.
So instead of making your cards look ancient and weathered, go for a look that would be authentic to the materials upon which artists worked in that era - perhaps a parchment, perhaps even an animal's hide! But make it appear new, as if the art was magically transported from the Renaissance and -POOF- turned into playing cards in the 21st century. Use great detail - but not excessive, because remember that your "canvas" is a mere 2.5 inches wide and 3.5 inches high! Use "pigments" authentic to the period, but bright and new, as if they were applied just yesterday and haven't yet fully dried.
Manufacturers - Expert Playing Card Company and Legends Playing Card Company have some of the best work around and at reasonable prices. MakePlayingCards.com is good if you have to go make cards on a budget, but their work isn't as good because of the use of digital printing rather than offset printing - it doesn't reproduce as sharp and colors tend to reproduce a little dark. I will say, though, that because of their low pricing, not only are they good for a budget deck design, they're a frequent choice for artists looking to print with another company but wanting some relatively inexpensive demo decks - proof-of-concept copies of the deck that allow you to get some idea of what the finished product will look like without having to spend a small fortune on demo decks from printers. (I've heard that USPC is now charging upwards of $500 for a test printing of a finished, cut deck, and it's printed digitally, not much different from how an MPC deck would appear. The same deck from MPC is something under $20, depending on the deck's features.)
USPC does do decent work - and you can't go by "gsm" weights when dealing with them, because they no longer use grams per square meter for their stock, instead using a range of caliper thickness. Their stocks are good - perhaps not as bad as being able to see light through them, since they are layered with graphite in the glue between the paper layers to increase opacity. Maybe the deck you saw was a counterfeit copy of a USPC/Bicycle deck? They are frequently forged by low-end Chinese printers looking to make a quick buck and sell cheap decks to inexpensive discount retailers and "dollar stores." As far as preference on thickness, that can really vary depending on the application. Some people prefer a heavier card, some a lighter card, some want stiff and some want more flexible. Consider who your audience is first, and determine what kind of card they prefer. You might even find that the target for your deck is more interested in the quality of the printing over the paper's properties - they might not want the cheapest paper out there, but they'll be happy with a mid-range paper as long as the artwork looks great. This is important to know - high-grade papers cost more money but might not necessarily sell more decks for you in the end or might push the cost of the deck too high for the audience you wish to target.
Best of luck to you!