I've used Dell computers at home and at work and like them a lot. For about $450, you can have a Dell machine that is plenty enough for gaming (Intel Core 2 Duo E7200, 2 GB RAM, 160 GB hard disk) then add a $100 graphics card from, say, newegg.com. That's perhaps competitive with building it yourself, and probably cheaper than building it yourself if you figure in any cost whatsoever to your own time.
Over the years, I've used Dells, HP's, Compaqs, Gateways, IBM's, and custom built and have been in on buying decisions for about 300 computers over time. I like Dell the best.