I have always built my own computers. Takes all of 2 hours or less once you have all the hardware shipped.
Current rig:
OS: Windows 7
Mobo: MS-7522
Processor: Intel i7 2.67GHz
RAM: 12GB
VideoCards: 2x Geforce GTX 460 SE
Hardrives: 2x 500GB 7200RPM 1x 1TB 7200RPM
Power Supply: Corsair 1Kilowatt
Only foreseeable upgrade that would pack the most punch on performance is swapping out 1 hard drive for a SSD and reformat my OS onto it, and dump all my most played games onto it to get those sweet 5second system restarts and instant game loading speeds.
When building computers only recommendation is to shop around, look for the best deals, and save yourself some heartache by paying extra for reliable RAM. The single most common hardware failure is RAM and hard drives. Nothing worse than waiting a few weeks for the goods only to get RAM that is dead on arrival.