I want to know if I can trade in my PC laptop (less than 6 months old) for a more powerful desktop.
I'm thinking 3ghz min, but not sure if that is too much. Decent graphics, and some good hd space.
Would it be possible to hook up a base unit to my laptop and use the laptop as the monitor?
You don't need 3Ghz nowadays. A great decent priced Quad-core CPU will clock in at around 2.6Ghz, remember, that is essentially (in a very basic terms) 4 CPU's running @ 2.6Ghz.
Seriosuly, take GSG's advice and give some thought to AMD's latest set of CHEAP quad-core chips. Sure the Intel's trounce them in every test, but the AMD chips are a quarter of the price for 80% of the performance, and that can't be beaten.