Help choosing new gfx card. I'm confuzzled!

Hey folks!

I am in the process of buying parts for an upgrade and I'm really stuck on what to buy GFX

wise I have about £150 to blow on the card itself. My current setup is as follows:

DFI LanParty nf3 250gb
AMD Athlon 64 3400+ Newcastle
2 x 512MB Corsair XMS Pro CMX512-3200C2 DDR SDRAM
Gainward GS7800+ Golden Sample 512MB AGP 3.0

I have so far brought (memory coming soon):
DUAL CORE INTEL PENTIUM D 945 CPU SOCKET 775 3.4Ghz
DFI Lanparty UT ICFX3200-T2R/G (Chipset is ATI CrossFire Xpress 3200 for Intel / ATI SB600)

So what for the graphics? I have always been an nVidia man, but with this mobo there is a

possibility of using ATI Crossfire which sounds exciting. I have read and tried to understand

the pros/cons of using Crossfire compared with a single GFX and have come out totally

confused!! What should i do go the nVidia 8800 route, ATI HD3870, or a couple of slower ATIs

in Crossfire?

I am looking to play the following games:
Crysis
Bioshock
Tabula Rasa
World of Warcraft
UT 2008

My monitor only goes up to 1400x900 so I wont be playing in any resolutions higher that this.

Your ideas are greatfully received