since you are using an excellent water cooling kit,
i thought it would be a great idea to get hold of two Water cooled GTX 480
now i realize this may not be able to trump a pair of HD5870(in crossfire) or a single HD5970 by a great margin,
but this setup has some added features like :
PHYSX(which should come in handy for enthusiast gamers in games like mass effect, batman etc.)
and
CUDA(not of much importance,but comes in handy while folding or hacking or image rendering)
and not to mention
better tesselation and performance when anti-aliasing is enabled compared to a pair of HD5870
which i feel would make your pc relevant for a longer period of time!!
in my personal opinion ATI cards are great VFM performers(i am planning to buy one myself!)
but when you are considering a pc of enthusiast budget(200k-300k), you shouldn't look beyond pair of NVIDIA's GTX 480
as for the extra heat produced:-you are getting a water cooling setup
extra power requirements:-get any 1000w psu from corsair or seasonic
P.S.
please excuse me if any of the information i've provided are incorrect
i am just trying to help
all the info i've provided are to the best of my knowledge