I have my eye on the "XFX GeForce 6200A 128MB DDR2 AGP8x". How much of a performance boost should I be seeing, if any?
Well, that card is now around 50$?? if you say you do play on medium settings in games released before 2003 even a FX5200 with 128mb will do the job, i think, and will be definitely cheaper
Now, i really think the card you have thought about is more than fine but if you can get the 256mb version which will be around 60-65$ it would be perfect. Have you thought about some models from ATI like the 9250 or 9550?, i think you can still find them below 70$. Keep in mind that i'm recommending this cards only if you want to save money.