The only reasons to buy a pre-built PC is if:
a) You're not gaming OR
b) You want to game but you buy a pre-built because you want a warranty on the entire system (not just the parts).
If you are willing to build your own, my only caution is to please spend some money on quality parts (don't cheap out on the PSU for example). If something goes wrong when you build your own PC, you have to find the part that failed and ship it to the manufacturer. With a pre-built, you just ship the entire PC.