Be Quiet's Straight Power E8 400W is a non-modular power supply coming at a very aggressive price point. According to the manufacturer it boasts an impressive 89% efficiency, also the unit is very quiet - no surprise, given the company name.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips