decent gaming computer cost? Right now my my laptop is better than my desktop :V

So I'd want atleast an i7 4gb of ram and a nvidia graphics card, not ati. To build from scratch or to buy online, whatever is cheaper.

OR would it be better to just get a new mobo and everything new for my current comp? I say new mobo cus it's a compaq prebuilt and I cant overclock otherwise I'd just do that >.<

~Please use more descriptive titles in the future. Thanks!~
DJ