edjcox, I'm afraid the amount of video memory on your card is in no way an indication of how fast it will perform. It determines what resolution textures you're able to run, and provides the GPU with some "workspace" to do its rendering. A very common "trick" of many vendors is to trumpet a system as "Kick-ass Gaming PC!!!", due to the card having oodles of RAM, but it's a fudge.
Unfortunately, the 5700's GPU, the part that does most of the donkey-work, is now very dated, being what, 4? 5? years old? Also, the 5-series cards, one of which I am currently using on this PC (5900 ultra), were never that great or efficient anyway, and had trouble running their own Pixel-shader 2 architecture. If you really want to upgrade your current card without changing or upgrading the whole PC, I'm sure there will still be a few AGP versions of ATi X800 pros and X850 XTs doing the rounds on E-bay for a decent price, and if you ensure it's a new card, you'll be safe to buy.
Like Grothesj2 said though, you'd also need to upgrade your PSU, as the cards I mentioned need a 350/400 Watt PSU, with a dedicated 6-pin power line for the card, which most good PSUs come with these days.
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