New PC , need help

Hi, I'm looking to buy a new rig for gaming, mainly games with High visual like Crysis, H.A.W.X, NFS etc.

Im on a budget though of +-1000 pounds

GPU:
HD 5870 , HD 4870x2 or Geforce GTX 285
I've seen benchmarks on this site and it looks like the HD 5870 is much faster than the other 2 cards
but which card is better for PhysX games like Cellfactor, Warmonger UT3 PhysX mod etc.?

My gut tells me I should go for the HD 5780

CPU:

Intel i7 920 or Phenom ii x4 965 BE?
how do they perform stock v stock, with no overclocking on either, when playing high visual games?

I've seen that you can overclock the i7 920 to run much faster than the phenom 965, but what cooling do u need to OC the i7 920 safely?,
e.g what CPU heatsink? and fans? case?

Just checking you can run a AMD processor with a nvidia GPU and Visa versa Ati and intel? (sorry im a bit new hardware atm)

RAM:
Transend aXeram™ DDR3 @ 1880 Mhz or Corsair XMS3 6Gb Tri-Channel Kit (3 x 2Gb) PC3-12800 DDR3 1600Mhz Intel i7 XMP Ready Series Gaming Memory (CL9-9-9-24; 1.65V)
both of these between 100 and 200 pounds

what other RAM would could you buy between 100 and 200 pounds, which might be better?

**What powersupply could one buy to safely power this Rig?
** whats the best Motherboard to buy, when maybe looking at Crossfire or SLi later, but for a cheap price

I have 2 HDD and one DVD drive

thanks in advance

Hi, you say GTX 285? but the HD 5870 gets better FPS, from the benchmarks i've seen and it's still in beta drivers