* The 2.8 P4 should be more than enough steam through BF2
* Videocard-wise - I'd recommend an AGP version of the X800/X850 or 6800 series. The X800/X850 have been seen these past couple of weeks under $200 w/rebate (
Like this one that sold out...X850 for $179). The 6600GT is a nice card...fast too, but the X800/X850 or 6800 chip will plant you in a different grade.
* More RAM....for BF2, there's definitely a difference in performance by going up to 1GB (you could go to 2GB and still get some gains...but not quite as much as you will from getting out of that 512MB range)
* Power Supply - kind of like the Spiderman slogan..."With great power comes great...power usage" You're probably already at 350-450watts on your power supply...but pay attention as you upgrade, if the system acts up, look to replace the power supply to a higher end unit.
Motherboard/AGP - yes, the PCI-E's will outstrip the AGP-based video cards. And they're already starting with the X1900's and the 7800 GTX's...The AGP-based 7800GS may be the final hurrah for AGP. If you go with just upgrading the video card to some high-end AGP card, I think you'll be happy for a couple of years until it's time to replace the entire system anyway...and go with that's new at that time, such as the new PCI-Marpat or whatever they'll call it.