In general: Go for a desktop. 4-6 gigs of ram ought to be good. 500-1000gigs of hard disk space is sufficient for most people. Get a nividea graphics card 340 series or greater. When buying a processor, aim for 2-4 cores.
Try not to get premade. Stay away from Alienware, and if you're getting a premade laptop, go for Asus.
Always avoid Acer as if it's the black plague.