What are you upgrading for? Also can you tell us the model of the motherboard if possible? You'll have to open up the case.

A couple of really important points first.

1. The 1156 pin (p55) boards do not run crossfire/sli particularly well as they don't have two full lane PCI-E 2 connectors for graphics cards. So if you want to run two GPUs either you'll have to get an HD 5970 (2* 5850 already) or upgrade to a 1366 pin based X series motherboard and CPU or sidegrade to an AMD system.

An AMD system would be something like an AM3 motherboard with a Phenom 2 965 Black edition. Maybe you ought to check the relative price of a motherboard and cpu locally? I don't know of which site you're using to get your prices from in the U.K so I can't help here.

2. Running two graphics cards in tandem is extremely hot so you'll have to make sure you have a large case and plenty of cooling. However this can be noisy and the system can easily be twice as loud with two cards as with one. You'd need to make sure the system was at least a mid tower, probably a full tower as you'd have to route a lot of cabling around.

As for your component list, it looks pretty good. If you're not happy with Lian Li you could consider an Antec P183 as an alternative, its quite quiet. I have one myself and its great. It looks very professional and striking. I've fitted an HD 5870 into it to check and whilst I had to remove a HDD add-on bay, im not going to use more than 4 HDDs. http://www.silentpcreview.com/antec-p183

I would also change the wireless to N specification. Its got twice the bandwidth and its on a less contested frequency so that ought to give you much better performance.

Your PSU choice depends on what graphics card you end up with, so I suggest that you make it the last component you decide upon.

Your GPU needs depend on what kind of monitor set up you'll be using. How many and what size. 30"/24"/22" etc.

Oh I won't have to open it... since I know it from memory :D

It's an old Abit IP35-E... but now that I think about it, I'm pretty sure it doesn't has compatibility with Intel's newest socket 775 processors... but I'm sure it is with older models (that I'm planning to buy for cheap on eBay)

Anyway, I want it for the usual gaming, plus 3D rendering, CAD, image editing, and what not. Quad core would be better than my current? Or would it be worse?

honestly chapset, I'd recommend directcanada or ncix over newegg.ca for the vast majority of purchases. Newegg.ca ships from the US, and you could easily be on the hook for customs charges and taxes when your stuff crosses the border. Not to mention both dc and ncix offer free shipping if you spend over a couple hundred unlike newegg. Buy from a Canadian site so there are no surprise costs.

www.directcanada.com
www.ncix.com

Thanks Squilliam!

I may just go for a single HD 5970. It's around £50-100 more than 2* 5850's, boasts a couple of FPS less than 2* 5870's CF (According to a few Crysis Benchmarks) and it seems it would save me the hassle of switching Motherboards, purchasing a higher watt psu and it should run cooler, quieter and free up some space. A 802.11n wireless card is pretty much useless for me at the moment as my router does not support that frequency. 

For a Monitor I'll probably go with a single 24 inch 1920x1200. The 2560x1600 resolution monitors I have seen are way too expensive. I'd use my 37 LG LH7000 TV but I have to sit to close to see any of the text.

Revamped List:

Motherboard: GIGABYTE GA-P55-UD3L LGA 1156 Intel P55 ATX Intel Motherboard
CPU: Intel Core i5-750 Processor
Case: Antec P183
GPU: ATI RADEON HD5970
RAM: Crucial memory 4GB Kit (2x2GB) DDR3 Ballistix PC3-8500 RAM 1066MHz CL = 7 Unbuffered NON-ECC 1.5V 256Meg x 64
Hard drive: Samsung SpinPoint F3 Desktop Class 1 TB Internal hard drive
Optical Drive: LG CH08LS10 Super Multi Blue
PowerSupply: ?
Operating System: Windows 7 64-BIT.
Wireless Card: EDIMAX EW-7128G IEEE 802.11b/g PCI Wireless Card

Thanks again Squilliam! I'll do some further research another time. For now it's 3:14am and my birthday, got to be up early!

Hi there, I am currently buying parts for a PC and, being from Vancouver, the first place I looked is NCIX.  Sadly they are considerably more expensive than Newegg.ca, although I am considering sending them an e-mail and asking them to price-match.  I'd also like to point out that Newegg.ca claims you won't have to pay any additional charges for the materials crossing the border. It's all included in the checkout cost.

Dunno about that Direct Canada site, its rather hard to navigate imo

Hi there, I am currently buying parts for a PC and, being from Vancouver, the first place I looked is NCIX.  Sadly they are considerably more expensive than Newegg.ca, although I am considering sending them an e-mail and asking them to price-match.  I'd also like to point out that Newegg.ca claims you won't have to pay any additional charges for the materials crossing the border. It's all included in the checkout cost.

Dunno about that Direct Canada site, its rather hard to navigate imo

NCIX is the place to go to for Canadian buyers, if only for one reason:

THey will pricematch any legitimate price from any online competitor in Canadian dollars. Period. It's more legwork, but you could end up saving two or three hundred bucks depending on what kind of rig you build.