I would like some help

Ok As of right now ive playing playin wow on this piece of crap specs:

Intel® Celeron® 900 (2.2GHz/800Mhz FSB/1MB cache)
2GB Shared Dual Channel DDR2 at 800MHz
Integrated Intel® X4500HD Graphics Media Accelerator
15.6 inch XGA display (1366x768)
130G HD
4-cell 2.0AHr Battery

Basically everything is intel integrated and i max at 25 fps and i run at 12 most of the time.

I want a desktop currently for gaming. I am wondering if i should buy a default desktop then add parts to it? Or just off the bat start customizing? I figured buy a decent board and find the best parts for it.

Also idk which is better Intel core i5 or core 2 duo,intel Pentium Dual core.