Building/Buying a gaming PC

Thanks for the recommendations guys, I'm goin' to bed, but I'll check 'em out in the morning.

I just wanted to say, forget about the extra money I had, I forgot to get an OS.

http://www.newegg.ca/Product/Product.asp...-_-Product

I'm confused about the whole "for system builders" thing, but I'm assuming it doesn't really matter. Could someone clear this up?