Don't overlook the fact that the OP's MX is either AGP or PCI. There was never a PCi-e MX to my knowledge. You can't just randomly go out and buy a new card unless you have yourself at least a partially modern mobo
Step 1: Find your motherboard make and model number. Assuming you're using XP with those specs, so grab
Everest Free, and do just that.
Step 2: Google your motherboard make and/or model number and check its specs. Does it have an AGP slot, or only PCI slots? Once you figure that out you know what cards you can fit in your machine.
Step 3: Go to whichever online retailer you use and find graphics cards with the interface you require (AGP, PCI or PCI-e) then list the ones you can afford on paper. Make sure they meet or exceed the minimum requirements.
Step 4: Head to Tomshardware to check the benchmarks of the cards you picked. Some of the newer cards aren't listed here, so you'll have to do your own searches for benches if that's the case. Pick the best performer on your list.
Benchmarks LinkStep 5: Buy it, slot your new card in, play (assuming you meet the other requirements)
Ask for help if you need it or get stuck. Don't guess - that never goes well

Cheers,
Mopy