Building new rig need help choosing

hello there
I know it looks something simpler to others, but in terms of building a computer for a purpose and budget its not..my need are different to others.
My price is £850 maybe £900 with a stretch.. I need help in deciding what GPU to buy for my new system in other words having ATI or Nvidia, the two cards in question are the HD4870 and the GTX280 (most likely the point of view or gainwood version) I think that they will both will do their job well. But I hear different on the ATI front...<_<.. anyways this is my spec that Im considering so far

Asus P5Q DELUXE or Maximus II Formula(sorry about spelling) I might go for a X48 mobo within the price range of £170 but not with DDR3 because I will go over budget
Asus silent knight 2 -that should be adiqute cooling for a q6600 oc
OCZ DDR2 1066 Mhz memory (4Gb)
SATA2 seagate 250Gb hard drives on raid 0 config
intel Q6600 oc to 3.2Ghz
thermltake tough power 750watt
thermltake armor

I just need help on choosing GPU and someone to put my fears of having problems with the HD4870 to rest

Yeah the HD x2 was on the list but I can't afford it even if I down grade to P5Q deluxe

Well its a sort off multi purpose gaming computer so I ll will be suing it for DVDs and multimedia stuff but also I want it to have enough firepower to play games like crysis on High at a good gamers resolution, my budget is no more than £850 for everything apart from PSU because I have that already and the OS. I hear that ATI have a texture problem, doing strange things in other words, but i m sure its not the card or the software its just that the poor thing is overheating. that is whats stopping me on deciding... I m not a big fan of cards like the GX2 and X2 for reasons but they are a tempting choice if I can afford them.
my powersupply shouldn't be a problem I will be running a 750watt tough power anyways