First of all, just to correct something - don’t get the Rage Pro. The equivalent of the TNT family is Rage 128 or Rage 128 Pro, not Rage Pro, which is older and considerably worse for OpenGL. But I couldn’t find a cheap Rage 128 PCI card on pricewatch, so this doesn’t really matter.
Checking pricewatch, the lowest cost 8MB TNT family card costs $27+shipping, the 16MB costs $41+shipping. 32MB costs $53+shipping (shipping does vary considerably, so check it out). So the price difference between 32 and 16 is smaller than for 16 vs. 8, for some reason.
8MB can get a little cramped, since at the very least you need to keep a front buffer, buck buffer and Z(/stencil) buffer. For example, a 1024x768 resolution with 32 bit colour takes 9MB - more than an 8MB card will support. But 800x600x16bit will take about 2.75MB, leaving you quite a bit of space for textures. But you only have hardware stencil buffer support in 32 bit mode, which could be a problem.
So basically, 16MB will enable to free you from doing calculations such as the above, while 8MB will be enough if you’re willing to limit yourself.
In general, I’d say that the 8MB version will be good enough. The reason is mainly the computer you’re using, which is definitely not a very good development computer. Most of what you’ll be doing will likely be writing small programs, and you probably won’t mind using low resolutions (especially considering that you now have a 2MB card, AFAICR, and probably a small monitor to match). So the limited card shouldn’t be that much of a problem.
Once you upgrade your system, you’ll probably want to get a better AGP card, anyway (and they are cheaper than PCI - $48+shipping will buy you a 32MB GeForce SDR). You’ll lose a little less money by going for the cheapest decent accelerator now (the 8MB TNT).
That’s just IMO. I always think like this when I buy my own equipment, and end up being tempted by more expensive stuff, and buying that instead of what I planned
But basically, since $200 or so will almost buy you a new computer, I think that you should either go for that, or go for the minimum. Next time, just save a couple hundred dollars for a rainy day