The FX 5200 would not be that big of an increase from your current card. The FX 5200 is known as the worthless card of the new budget cards that NVIDIA released. You can get them online for $30 without rebates. Trust me, they're pieces of shit. Not to mention the one you found isn't even an Ultra, even though they aren't worth getting either!
If you truly want a good graphics card for $100, buy a GeForce 4 TI4200. You can find them on the Internet, but you won't find them in-store anywhere. Please, please don't buy the FX 5200.
Check out
www.newegg.com and see if they have any TI4200s in your price range. Go for the 64 MB version. Good luck!
-Lasereth