Help building first gaming PC

[Pyromaniac]

Hi all. I have never built an entire PC before, but i have done some upgrading and i think i could just about build one now. Anyway, i have been asked to build a gaming PC for my friend for about £500 (not including labour) and i need some advice on what components to get. Here's what i've come up with;
AMD-Athlon-II-X4-620-Quad-Core-S-AM3-26GHz-2MB-Cache-HT-4000MHz-95W (http://www.scan.co.uk/Products/AMD-Athlon-II-X4-620-Quad-Core-S-AM3-26GHz-2MB-Cache-HT-4000MHz-95W-Retail)
Is the quad core really worth it or should i save some money here and get the dual core one?
I'd like to get a graphics card to match the performance of the processor but i don't really know how good that processor is? I like the idea of running 2 ATI hd4770's as i heard from tom's hardware that these perform outstandingly in crossfire. Would this just be overkill with that processor though? The other option would be to go with a single card like the 4870 or 4850, or perhaps an Nvidia card? I really don't know.
Obviously i need a mobo to match the processor and have sufficient slots for two graphics cards if i go with two, but other than that my knowledge on chipsets and other things is almost non existant, so i need help in this area too.
I don't want to go too far into the more secondary components yet, but i'd like to get a basic idea of how much the RAM for this system will cost. Ideally i'd like to match the RAM to the processor, so what type and speed would i need for this? And how much RAM is enough? 4GB?
Lastly could someone explain motherboard types and sizes so that i can figure out what kind of case i should get.

Sorry for the mega long post, and thanks in advance for any advice.

Luke

[Pyromaniac]

Woops, didn't mean to post this in this section :eek: . Can one of the admins please move it to the right section.
sorry.
;)

[Pyromaniac]

ok i'll just repost in the right section and if an admin comes across this could you please delete it?

[PrntRhd]

No need, I moved it.

[fincheylv]

You may find some of the info in my step-by-step guide to picking parts and building a pc (http://www.squidoo.com/building-a-pc) useful.

[zvermm]

If you're trying to decide between a 2 cheaper GPU setup or one newer and more expensive, my advice would be to go with the single, higher class card. From my experience, SLI or crossfire are only worth it if you got money to burn and can afford 2 high end GPUs and the monster system to go with them.
Several reasons:
First, cost to performance. Not worth it. You're not getting x2 the performance when you buy 2 GPUs. The best I've seen in tests online was about 40% if I remember right. Better buy one better card.
Second, plan for the future. A newer card uses newer technology and will serve you much longer than 2 older cards. Currently it's a bit ludicrous with Nvidia having physx versus ATI's DX11 (why can't we have both..?).
Third, stability. My one system that had crossfire was pretty high on glitches and graphical errors. Keep in mind that many games and applications don't take well to that setup and you'll be disabling it more than you think.