what grafix card / cards should i buy

[THE_ASSASIN]

hi all i am just in the prosses of upgrading my gaming pc and now i would like to upgrade my grafix card i have £500 to spend what it the best option 1 or 2 cards and which card /cards should i buy

mobo is asus maximus 2 formula
cpu 3ghz core 2 duo e8400
cooling thermaltake 760i watercooling+extra drive bay radiator+themal alien water cooling cpu block and i will be watercool my north b and south b + grafix cards +memory
memory 2x2 2ghz 800mgz ddr2 corsar
card i have now is 8800gts 640 meg ddr3
thanks for any input / the_assasin