P4 550 = too hot, Athlon 64 (regular/X2/FX) CPU's are better gamers.

Pentium D 805 = a nice overclocking project if you have the time and the money for high quality components...and are willing to replace parts that you might fry. (I take it that you read the OCing article.)

Although I'm not the biggest gamer, I try to keep up with the technology. I'd put the money into a video card with a good GPU.