£850 Build Idea - help narrow cost down

Change the case to something like the CM HAF 922, CM 690 II Advanced, or Antec 902. And you should really get a better power supply. Seeing as you can't crossfire on that board a 500W unit from Antec, Cooler Master, Corsair, OCZ would be fine. If you want to be able to take the PSU into your next build then a 650W from a good brand would be fine as well.
The only other way to save costs is to get an AMD Phenom II X4 955, and then you could spend less on a single PCI-E slot mobo or the same amount on a crossfire capable mobo.

Do i need a 6gb motherboard since i am using 4gb of ram?

I am looking for an alternate powersource/case now

Sorry, I don't know what you mean here. The RAM you have atm the moment is fine, and the motherboard is fine as well.

Updated, how does this look?

Costs an extra £7 but i added abetter charger and a case which is better forair flow

power supply not charger

You could save £23 switching to Corsair 450VX power supply seeing as you can't run SLI/CF anyway. Your system really should only pull about 350W so maybe even the 400CX will be enough saving you £42.

Yea, the only thing possibly wrong with that build is that it can't crossfire and the PSU is too powerful. But crossfiring capability can't be added without adding £50 or changing to an AMD system. And the PSU is high quality and I'd say very likely to go into the next build.

what will i need to change to be able to crossfire?

If you stay with the Intel i5 750, just the motherboard. On that site it would probably have to be either of these:
Asus P7P55D Pro £143
Gigabyte GA-P55M-UD4 £139
Gigabyte GA-P55-UD5 £142
Which as you can see costs more. Unless you are running a monstrous resolution then you will not need another 5850 for awhile. I suppose it is nice to have the option to just buy another card rather than be forced to replace it.
If you decide that you still want to be able to crossfire and don't want to pay more, then you can get an x4 955 which is around £125 and then the Asus M4A79TD EVO which is around £90. And the Phenom x4 955 is a very nice gaming CPU.

I'm actually a single card kind of guy myself, so I think your build is fine the way it is. I'd certainly be happy with it.

The motherboard.
It needs 2 PCI-e slots at x16 or x8.

Think i will stick with the motherboard i've chosen. Since i won't be paying another £300 for a second graphics card.

If i overclock to like 3.8 - 4ghz will i need a 600-650w PSU?

Probably not.

500w seems sufficient?

Yea it probably would. The way I've calculated it is. 150W for a voltage adjusted overclocked i5 750, 150W for the 5850. And then 100W for everything else/margin. Which totals 400W. So on maximum load it would be 80% of a 500W unit's rated wattage, which is a decent number to be around, altho higher is not desirable.

thank you just pricing parts upin differentr sites. You solved my problems

If you are going to be overclocking then it would be wise to invest in a CPU cooler. If you don't have one already then the Coolermaster Hyper 212 Plus £20 is one of the best and most worthwhile coolers.