just google for pc building guides. there are a few guides pinned on this forum as well. any difficulties just post a new thread with your troubles and there will always be someone here to help.
BUD dont buy a pc from one of those builder sites, you will get screwed, prices are high, and i bought on and you can build a better pc for around the same price, first the ps on that case wouldnt be sufficient to run SLi, you def want to upgrade that vid card, i have a msi 6200 256mb and it wasn't great, so i bought a 7800gt. i wouldnt run sli anyways because its so bloody pricey and at most only a like a 10-30% increase in quality at least a 200% increase in price. if u want a 6200 build this pc and ill sell u mine cheap, im not going to use it.
this is definately a decent set up w/o vid card because i dont know how much u want to spend, but i wish i just built my pc it would kick. and if u want to go cheap go for it, but these parts i know are definately good
If you know exactly what parts you want, then its all about getting them parts for the cheapest price. try ebay for some last minute bargains on hardware.
When you have all the parts you need check out www.buildyourown.org.uk
for advise on how to put it all together.
Ask some of your friends if they know anything about how to put a PC together, you might be surprised.