first of all, i would strongly reccommend you build your own
you save money, you get to choose the parts, and you learn something about computers
if that isn't an option, then you should buy a prebuilt
Compaq: Pretty cheap for prebuilt, but isn't the most reliable (if i hear things right)
HP: Good comps, haven't heard anything bad
Dell: Good comps, bad tech support, hardER upgradability
Gateway: dunno
i would also look at cyberpowersystems.com and ibuypower.com, but i have heard of people who get damaged comps from those sources
and btw you aren't going to get a gaming comp for $350-$450 if thats your intention