I'd say there's still at least a few thousand people on at one time still. It's never hard for me to find a populated server. I play BF2 at least every other week. It's still my fav FPS game of all time and I can not wait til play4free and BF3 .
I'm putting together zero dollar BF2 system for my brother. It will run Windows XP. Other than BF 1.5 patch, is anything else required, e.g. Punkbuster update?