For basic noise reduction, it's probably best to replace the cpu heatsink, and possibly get a fan controller for the case fans. If you've got any left after that, and don't mind voiding warranties, then replacing the vid card fan usually helps.
As for the Dell, first of all, Dell doesn't make CPUs. A Dell computer on the other hand can probably be built cheaper if you buy parts separately. At the very least, buying parts separately can allow you to know and get exactly what you want.