From your post, is not clear if you want to overclock, or running SLI/crossfire.
I avoid "this week only offers", although there are good deals:
Withouth SLI/crossfire
Processor: I5 720
http://www.overclockers.co.uk/showproduct.php?prodid=CP-301-IN&groupid=701&catid=6&subcat=1272
Motherboard:Gigabyte GA-P55M-UD2
http://www.overclockers.co.uk/showproduct.php?prodid=MB-209-GI&groupid=701&catid=5&subcat=1495
AVOID ANY INTEL!! Intel motherboards have many issues and annoyances, and poor drivers.
Memory: G.Skill Ripjaw 4GB (2x2GB) DDR3 PC3-16000C9 2000MHz (F3-16000CL9D-4GBRH)
http://www.overclockers.co.uk/showproduct.php?prodid=MY-037-GS&groupid=701&catid=8&subcat=1387
A steal
Alternative:Patriot Extreme Performance 4GB (2x2GB) PC3-12800 1600MHz (PDC34G1600ELK)
This is £15 cheaper, meaning that you will see a little reduced performance on rare programs, as Winrar/Winzip, but no difference in gaming.
With SLI/crossfire
Here you need the X58 platform, because is a lot faster for multiple video cards.
Processor: I7 920
http://www.overclockers.co.uk/showproduct.php?prodid=CP-265-IN&groupid=701&catid=6&subcat=1272
In some applications is faster than the i5, in others slower. Not much of a difference, but is definitely faster in SLI/Crossfire, by a good amount.
Motherboard:Gigabyte GA-EX58-UD3R
http://www.overclockers.co.uk/showproduct.php?prodid=MB-159-GI&groupid=701&catid=5&subcat=1283
Out the box only supports crossfire, but with bios update also supports SLI. Is also OK for overclocking.
Out the box only supports crossfire, but with bios update also supports SLI.
Alternative: Asus P6T. A bit more expensive, but includes a third PCI-E slot which may become useful if you plan to conserve an old Geforce as PhysX processor, or buy PCI-E SSD storage. Also, is a better overclocker, but no much.
Warning: the Asus P6T SE is another model, without SLI support. No problem if you want crossfire.
AVOID ANY INTEL!! Intel motherboards have many issues and annoyances, and poor drivers.
Memory:Kingston HyperX 6GB (3x2GB) DDR3 PC3-12800C8 (1600MHz) (KHX12800D3LLK3/6GX) 1.65v CAS8
http://www.overclockers.co.uk/showproduct.php?prodid=MY-052-KS&groupid=701&catid=8&subcat=1389
You gonna be fine with the cheaper 4Gb dual channel kits, saving 30 bucks, but Intel states that as soon as the software start using the 8 threads, triple channel will make a noticeable difference. We still don't know. Is a good bet that at the times that games uses all the 8 threads, much better memory will be crazily cheap, so if you have other thing to buy with those £30, go for it.
Cooler:
To use the memories at maximum speed you need to overvolt the memory bus to 1.65v (which intel states is safe), but this raises a lot the temperature of the processor with intel stock cooler, so is a good investment to buy a better cooler, whose main benefit is to allow overclocking to an extra 1 Ghz, which is safe, and gives at least 30% boost.
Zalman CNPS9900-NT CPU Cooler (Socket 754/939/940/AM2/775/1156/1366)
http://www.overclockers.co.uk/showproduct.php?prodid=HS-043-ZA&groupid=701&catid=57&subcat=1395
There are far cheaper coolers, like these:
Zalman CNPS9500A-LED CPU Cooler (Socket 478/754/939/940/LGA775)
http://www.overclockers.co.uk/showproduct.php?prodid=HS-015-ZA&groupid=701&catid=57&subcat=1395
but needs LGA 775 (Core 2 motherboards) holes on the mother. I know that the Asus P6T have LGA 775 and 1156/1366 holes, but don't know about the gigabyte ones. Do your investigations.
Good coolers have pipes in addition of radiators, and are silent.
Video cards.
Today the best ones are the ATI 5850/5870. They are the best performers, and the onliest supporting DX11.
The problem is that they does not support PhysX. Only Geforces does that, and around the corner there are the new DX11 Geforces G300. Is not safe to buy video cards until we know for sure real world comparisons between ATI and Nvidia in DX11.
I would not buy a new video card today, but if you need to buy one, the best investments are, in order of performance:
Radeon 5850
Radeon 5870
2x Radeon 5850 (crossfire)
2x Radeon 5870 (crossfire)
www.overclockers.co.uk appears to not have stock, so i do not include links.
the Radeon 5850 in the i5 is anything you need. I recommend to buy it, and save the money. If tomorrow you need the geforces features, then sold this card, and go for a geforce, or buy a second Radeon 5850 to do crossfire
Power
OCZ StealthXStream 600w Silent SLI Ready Power Supply
http://www.overclockers.co.uk/showproduct.php?prodid=CA-023-OC&groupid=701&catid=123&subcat=
It should be enough to run SLI/crossfire and some overclocking without adding too much noise to your system
Storage:
buy the cheaper 1 Tb HD, and a 64/120 Gb SSD to install windows 7 64 bit:
Samsung SpinPoint F3 1TB SATA-II 32MB Cache - OEM (HD103SJ)
http://www.overclockers.co.uk/showproduct.php?prodid=HD-082-SA&groupid=701&catid=14&subcat=1279
For SSD disks, stay with Indilinx or Intel controllers. Avoid Jmicron. Samsung does some ok, and some very bad controllers.
Intel is expensive, and have bad
Trim support and write speed. Indilinx is the more trustworthy, in
OCZ Vertex incarnation.
Unfortunately, they are still so expensive, that is good advice to wait to the next generation, until prices halves over the 100Gb capacities, and Trim support gets out of beta state.
SSD disks (the good ones. others are slower that magnetic disks) gives the most important end user experience boost. Not only games load faster, Windows boot faster, but antivirus does not slow down the system, and games do not continually freezes in random and annoying stuttering when windows start indexing the drives, fragmenting, swapping memory, or doing all those annoying background tasks.
This is my recommended investment:
OCZ Vertex Series 60GB 2.5" SATA-II Solid State Hard Drive (OCZSSD2-1VTX60G)
http://www.overclockers.co.uk/showproduct.php?prodid=HD-014-OC&groupid=701&catid=14&subcat=910
If you manage to collect more money:
OCZ Vertex Series 120GB 2.5" SATA-II Solid State Hard Drive (OCZSSD2-1VTX120G)
http://www.overclockers.co.uk/showproduct.php?prodid=HD-015-OC&groupid=701&catid=14&subcat=1427
But it may break your budget
Avoid RAID 0 setups, because drivers still do not support TRIM under RAID. I would procastinate the buy of SSD until Drivers get fixed with RAID, TRIM, and wear leveling.