buying a bundle from a store should be a safe bet.
building and modding can hazardous tasks.......and expensive.
both the systems u mentioned would be fine.........buy either one and demand that the store include a copy of CALL OF DUTY.
the amd system is cheaper because amd processors are in general cheaper tha intels........but take not the amd 2600 plus only runs vat 2.06 mhz not 2.6 mhz as it's name might suggest and has fsb 333mhz. i'd still plump for the p4 system.
all the best with whichever u buy...........don't forget the copy of cod
doddsy