Brand doesn't matter much, but IMO get a reference card because the non reference ones tend to blow the hot air around in your case instead of out the back. Also many non reference ones have voltage control disabled although that probably won't be an issue.
Oh right and you'll need an aftermarket cooler on that i5 for OCing
Well CF 5870s would be really, really sick. Honestly tho I can't see a game, even Crysis 2, giving CF 5850s trouble for several years. Not unless some super mega ultra graphics engine comes out. And if you OC the 5850s you can get a lot more performance than stock. Mine went from 700/1000 to 960/1210.
But hey, if you've got the extra cash, might as well go for CF 5870s. Although while you have 1 5870, you might find yourself having to lower some graphic settings to keep solid framerates (50+).
Honestly tho, bang for your buck CF 5850s are just awesome.
I had a single 5850 for about a month and it was pretty impressive, but then I added a second one and I'm blown away. Crysis, Crysis Warhead, Metro 2033, Dirt 2, NFS Shift, STALKER: CoP all on max settings and very nice framerates. I just don't see the point in paying any more when these already are crushing games.
And the RAM should be fine, but since you want to OC I'd recommend looking for 1600mhz CL8 ram. Stock bclk on an i5 is 133 so the ram multiplies by 10 for 1333. For 1600mhz RAM you raise the bclk to 160 - and generally won't need to change any voltage settings for this to work (on the CPU side, RAM will generally need 1.65V but that's all manufacturer specified). My i5 750 is running 175bclk, so the ram I have at 8x for 1400mhz CL7. It's very fast. And obviously the CPU is running fast, it's going 3.7ghz normal (x21) and with turbo hits 4.2ghz! (x24 for single and double thread operations). Just a thought tho, but reality is that RAM doesn't really affect performance... it's just good to have that headroom for OCing.
No, WoW definitely can be ran max settings. I mean games like Crysis or Metro will still be able to be on High settings just not the absolute max. WoW is fairly old and can run cards far below the 5870.