|1. 768MB isn't enough RAM for decent gaming...you should have 2GB.|
2. since PCI-e has replaced AGP as the graphics standard, there are very few "modern" AGP cards being made. nVidia has all but bailed out of the AGP business, ATI throws a bone to the AGP diehards once in a while (HD4650, HD4670) but unfortunately, the driver support for AGP isn't the greatest.
The amount of memory on the card doesn't necessarily mean it's fast. There are 256MB cards that will blow away 1GB cards. It all depends on the graphics chip, the width of the memory bus (64-bit, 128-bit, 256-bit), type of memory (DDR, DDR2, DDR3, etc), & the speed the memory runs at.
The Radeon X1950 for about $75 (if you can find it) is about as good as it gets. There are others better & faster, but they cost more & the driver support just isn't there.
3. a large percentage of gaming cards require a direct plug in from the power supply. This is because the AGP (or PCI-e) slot can't supply all the power the card needs. In other words, if you get a power hungry card & your power supply is too weak, you will have to upgrade that too.
To do it right, you need to invest in RAM, a gaming AGP card & possibly a power supply (quality parts, not cheap junk). It will cost you anywhere between $125-200 (online prices, not BestBuy) depending on what you get. Is it worth it on a system that old?