First time builder - gaming PC

Edit: Please read all of thread as half of this information is now incorrect

APPROXIMATE PURCHASE DATE: Next few weeks

BUDGET: Not really got a budget but hoping for ~£500

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Gaming, surfing the Internet, Microsoft only software.

PARTS NOT REQUIRED: Just looking to build the tower, I have a case from current PC but not sure if it will be compatible, also it lays flat and I would prefer a tower so it takes up less room.

PREFERRED WEBSITE(S) FOR PARTS: I use amazon.co.uk a lot and really like it, but I don't know how useful it will be and will happily shop elsewhere. (I'm in UK)

PARTS PREFERENCES: Not really, I'll just take your advice.

OVERCLOCKING, SLI OR CROSSFIRE: Whatever is best for performance/cost ratio.

MONITOR RESOLUTION: Need to get one of these as well but wasn't including in the budget, advice would be appreciated.

ADDITIONAL COMMENTS: I only have Windows 7, if that makes a difference. Will mainly be playing WoW and RTS games, as I have an XBox, I'm hoping this means I won't need to spend so much.

I just really need advice as there's so many choices and I don't know where to start. Also is it worth trying to sell parts of my old PC? That would mean I could spend more on the new computer.
Please tell me if I missed anything or you need more information.

Off Ebuyer UK

Hit at least a 16:9 20" LCD and game on ^^

Thanks for the suggestion, I'd almost given up. The build looks pretty good.
Also I have been thinking and discussing with friends, (and just got a bank statement) and I would to go up to around £1000 (would have to include monitor) if it is possible to get one which will last about 5 years (with some upgrades) and a year or so without needing to upgrade. This should make things easier at uni where I will be starting end of next year (and staying for 3/4 years), and will probably not be taking my Xbox. Any opinion on this is greatly appreciated, as I am certain you know a lot more than my friends.

Thanks for the input, looks good. I am presuming the crossfiring is for adding another card graphics card in the future?
I've been browsing the monitors for sale, and am looking for something like this: http://www.ebuyer.com/product/179807 (I need HDMI for Xbox)
I was wondering if anyone could recommend a with better value for money, as that would bump me up most of the way to my budget. Also I don't really understand all the specs.

I would go for one of those, the Philips is very good, the BenQ is LED backlit.

http://www.ebuyer.com/product/165288

http://www.ebuyer.com/product/199307

http://www.ebuyer.com/product/177131

I would suggest looking at cases, because you might find something that you like more.

I just noticed that the Asrock has the two fast PCI-E slots quite close to each other - the two blues ones. If you put a double slot graphics card into the first slot it would be right on top of the second card, which might cause some heat issues. Altho there are going to be heat issues with two graphics cards anyway.
If you want another option for a crossfire capable mobo, then this one is a bit cheaper than the Asus: Gigabyte GA-890GPA-UD3H 890GX £105

The only real requirement is that it is of the ATX form factor. I think it's beneficial in terms of temps to have at least one intake fan and two exhausts.

Any of these take your fancy?
Antec 300
Lancool PC-K58
Xigmatek Utgard
CM Storm Scout
Antec 900
Coolermaster CM690

I'm not sure I understand why the more expensive motherboard is better, whats a double slot graphics card?
Here's the two mobos:
http://www.ebuyer.com/product/197546
http://www.ebuyer.com/product/219204

Not really they are both IGP 800 series chipsets with the Giga having a better IGP which is irrelevant when one is going discrete GPU ^^ Here is an real owners' thread for the Asrock 800 series mobos
http://67.90.82.13/forums/showthread.php?t=247736

Okay thanks for clearing that up, and finally, which PSU is better? They both work out at around the same price.
I would guess this one, but I don't know much about PSUs, http://www.ebuyer.com/product/147047
this is the other one: http://www.ebuyer.com/product/191826

I drew a paint diagram to show you roughly what each configuration would look like. Basically there is more potential for cool air to get to both graphics cards on the Gigabyte than the Asrock.

The XFX is definitely the best one there, as it performs very well - very safe to use, is modular, is more efficient and is cheaper, especially if you can get it for £70.

Okay I guess I'll go with the ASRock then, thanks to all of you for the great help!