Is there a reasonable/cheap way to upgrade my PC? I would love to gain a few FPS in games / use it as a Steam streaming machine.
Motherboard: ASUS P5Q-PRO
CPU: Intel Core 2 Duo E7200
GPU: Radeon 4850
4 GB DDR2 RAM
Power supply: 400W, midrange (I don't remember the exact brand/model)
HDD: 2 TB WD RED
Yeah upgrade the GPU. As to which GPU, that'll depend on your budget.