Well,the 6600GT is more than capable of running bf2 in high settings but this also depends on the fact tht you should have a sufficient RAM like 2gb minimum cause ppl are experiencing laggy gameplay with 1gb while some don't.So it would be better if you have 2gb to play smoothly or like funky said..just tone down ur settings and you'll have smooth gameplay.
As in availability of players,there are lots but I'm afraid tht number will decline once the Quake wars mod releases,I have been advising my frnds tht unless their pc can handle bf2..don't buy it but rather go for quake 4 since it's mod gonna be released this year and it looks nicer than bf2.
But overall,I can say bf2 is awesome..but as you know EA style i.e. to drain your purses,they'll be releasing a lotta expansion packs which originally should have been released with the updates.Now there is Special Forces and Euro force,another one tht's gonna be released is armored fury.