hey evi, do you live in the US? If so prices will be much lower than most other countries. Price around
http://www.newegg.com and
http://www.zipzoomfly.com are a few good places to get your parts. Best Buy and Circuit City are rip-offs, don\'t even bother with them.
http://www.buildyourown.org.uk/it\'s not too hard really, anyone with any technical skills and will-power can put one together. And like you said, you save a boatload of cash and end up with an even better computer than a pre-built.
Plus I enjoy building it as much as using it. Gonna upgrade this fall again, probably only my CPU and Ram... I\'m really in need of 2 gigs for BF2. And for 2 gigs to stay dual channel you really need 2 x 1 GB sticks. Guess i\'ll just have to sell my current 2 x 512 on ebay or something.
A lot of newbies to building a rig make a wish list on newegg and post it for others to critique. That way you know that the parts you are buying are compatible and worth the money.