It's still better to build your own I think. True, pre-built PC's are much cheaper than they used to be, for the most part, but the parts used are crap usually and you also run into the problem of proprietary cases/parts etc. If you ever have to replace a part or upgrade something, it can be a nightmare to do so with one of their cases usually.
Sure, you can get a pre-built computer with a Quad core and 4GB of RAM for $800 (I just saw one from Acer) but the RAM is probably the cheapest stuff possible, the MoBo is probably an ECS P.O.S., and it would have a frickin 8600 GPU. Not worth it, since you'd want to swap out at least the vid card for something better and end up paying more than you would if you just built it yourself from the get-go.
Building yourself you can still save quite a bit of money, and at the very least you ensure that you're not getting crap parts. Of course, if you are looking at a Dell XPS or HP Blackbird, then yeah, you're probably getting a pretty good setup with AAA parts, but you're also spending $5000+ for a stupid computer. What is this, 1992?