Here is what I am thinking so far:

CPU - IntelЎ Core™ i7-950 Processor (8M Cache, 3.06 GHz) - Ѓ200

GPU - nvidia geforce gtx 580 - Ѓ336 for 1

Motherboard - Asus P6X58D-E - Ѓ150

RAM - Corsair DDR3 (6x2GB) - Ѓ135

Sound - sound blaster x-fi titanium pci express - Ѓ70

Cooling - Titan Fenrir EVO cpu cooler - Ѓ35

Power - Corsair CMPSU-850HX ATX12V & EPS12V Power Supply - Ѓ126

Case - Antect902 - Ѓ90

Haven't got a cd drive or networking yet, but that is cheap.

Post any improvements you can think of for similar prices

Harddrive - Momentus (flavour of choice)
RAM - make it 3x4gb sticks so it's easier to upgrade. More ram = more speed. Max that out bro

What about graphics card? The Motherboard supports 3 way sli, so should I be getting 2 or 3 cheaper cards or just stick with the single gtx580?

Get 3 460's

you cant use gtx 460's in 3 way sli. only in 2 way

Well, get 2 460's.

gtx 580 = sli gtx 460's for performance find out which is cheaper and takes less power and go with that one. also if you don't want the hassle of sli just stick with the 580. you would actually be fine with a single 570 for that matter but if you want the best of the best in a single card configuration the 580 is it at the moment as it surpasses even single card crossfire or single card sli ( hd 5970 or gtx 295) in a lot of benchmarks.

What are you using this computer for?

The 580 can handle any game with ease, and will do for at least the next 2-3 years.

Don't SLI unless you really want to, or unless you really need that power.

Harddrive and RAM are the 2 most important factors IMO.

I agree, having a faster hard drive and faster ram definitely makes a big difference in total performance and yea a single 580 will be more compatible than 2 460's as not all games support sli.

From the research I have done RAM is RAM is RAM and it mostly depends on how much you have. So you may want to try to find something a tad cheaper there, though most of it costs about the same.

Also I agree not to worry too much about the GPU. My Pc is bare-bones as fuck right now and I dont even have one aside from the on-board, and honestly is does a lot better than I thought. With the more expensive graphics cards you end up paying more and more for slight increases of power, when all most people need or will use is a nice midrange card that is solid. Future building is something to consider, but honestly with most games the experience of playing is just as fun without the graphics turned all the way up.

Example: I recently got call of duty, modern warfare 2. With the graphics on full it runs a little too laggy to play, but when I take them down I get fast, smooth gameplay. And that is without a separate video card.