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.
Currently running:
* Intel P4 550 3.4 GHz with HT technology and 64bit
* 1Gb PC 3200 Ram
Moving up to:
* Intel Pentium D 805 2.66 (overclocking with liquid cooler to 4.1 GHz)
* 2Gb PC 5300 667 Ram
Will I notice much of a difference on BF2?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic