Ok, I'll chime in...Sounds like you're pretty close to pulling the trigger & getting this build done...Plenty of great suggestions have already been made...So here's my .02...
You'll do fine with either a GTX 260 or 280...BF2 is pretty much the only game I play & both of my Rigs run the game like butter & max out at 100 FPS no problem...The 260 & 280 will at the very least match my cards in CF & SLI, & probably beat them in performance...I have no complaints whatsoever about my 8400's & I have both OC'd to 4 Ghz on air...Keep in mind, you'll need a good mobo to do that, so don't skimp on that (I only use ASUS MB's)...
I'm a RAM whore, so I recommend a bare minimum of 4 gigs...If you decide to go with Vista 64, you can go with 8 GB & it won't destroy your wallet...Personally, I have absolutely no interest in ever going back to XP, so my recommendation is Vista 64...One thing you need to know in case you do with Vista 64 is you need to run BF2 as Administrator & then run it in XP Mode...This is actually very simple to do...
So the breakdown is this...
CPU: E8400
RAM: 4 - 8 GB (DDR2 is MUCH less than DDR3 & DDR3 certainly isn't worth the extra coin)
OS: Vista 64
VGA Card: GTX 260 or 280 (Both will work great with BF2) Sidebar: If you don't want to need to upgrade for a long time & you can afford it, go with the GTX 295...
Motherboard: ASUS, EVGA & Gigabyte all make solid boards, but my preference has always been ASUS
Send me a PM if you have more specific questions & you aren't seeing the answers in this discussion...I'd be glad to help...