Well as for what card imo i would go with Nvidia i think they run better with less probs. saying that i have one of my other PC's with ATI X800Pro 256mb and had no probs with that. $120 you say well i am in the uk so i work that as about £70 i think

.I have had alook anyway and from what i can see on Nvidia AGP cards (i assume thats what your motherboard supports up to AGP) theres not really anything there worth getting tbh in the price range. So i would say go with ATI the one i can see thats best to get which would be a far leap forward from what your using is the ATI Radeon X1600 Pro 512MB DDR2 TV-Out/DVI (AGP) i found it for £99 but with the stupid VAT we have to pay over here i am sure you can get it for your price range.
New Card:
- Powered by ATI Radeon X1600 Pro - GPU 500MHz
- 512MB-128bit 4 channel DDR2 memory - Memory 800MHz
- 12 Pixel shader processor
- 5 Vertex shader processor
- 8 Geometry Pipelines
- Ultra-threaded SM 3.0 Engine
- 256-bit Ring-bus
Current Card you have:
-GPU 250MHz
-Memory 400MHz
-128MB DDR Memory
- 256-bit Graphics Core
- 4 Pixels Per Clock
- 16 Textures per pixel (max per pass)
- Dual 350MHz RAMDACs
Hope that helps