I have acquired an old HP DC7700 from my work. I've stuck a copy of linux on it and a WD Caviar Green 500GB hard drive. Luckily, Linux recognizes my wifi USB stick, so I am looking for new ways to enhance my machine.
The only candidate seems to be to replace the Nvidia Quaddro NVS 280 graphics card with something a little more fun. I doubt the motherboard recognizes PCIe 2.1 cards, and I am on a really tight budget, so I think the best option would be a Nvidia GT 430. The 440 draws a lot more power without giving that much more performance and costs twice as much. Also, the CPU is a 1.8 GHz pentium dual core so I suppose even a modest GPU would quickly outpace the CPU.
From other forums I have determined that my version of linux has fairly mature drivers for the 430.
I'm not looking to play the latest games - older ones like CS Source, Doom III, Left 4 Dead and so on (in Linux Wine).