I'm running a gateway cx210, 2ghz Core2, which blows away my old 3ghz p4 w/hyper threading.
I've had good luck with both Gateway and HP computers.
Also if you do anything with graphics, or take notes on your computer get a
convertible tablet. They're great.
DO NOT pay to have the manufacturer install more ram! You can buy it at as little a 25% of the manufacturers cost, and install it your self in about 10 min.!(If you do this its your fault if you break something, so don't blame me, but I've never had any issues.)
I do recommend you have 2gb if your running Vista.