It also depends on the board. Gen1 775 only support core 2 duo the gen2 boards support core 2 quad. Only know this because I had a 775, bought a Q6600 and it didn't work in it. So check you motherboard model on the net and you can see what CPU's it can support and what bios you need to be
One of my spare PC's is a 775 MSI P43 Neo, with an E8600 Core Duo, OCed to 4.18GHz running an EVGA GTX 550Ti lol. It's a good browsing PC/work horse. It might run counter strike haha
Modded xeon. I had one sat at 4.0ghz even though its old it still plays most games only thing to bear in mind the temp threshold is like 60-70oC whereas the core 2 quads can run stable at 80-90oC. Only way to get the xeon efficent is watercooling and aiming for a 60oC temp at full load