Sparkle, Enermax, Antec, and PC Power and Cooling are all well-regarded makes.

 

A good 300 watt supply is often better than a low-quality "400" or "450".

 

Note that depending on the motherboard you have, it might be worthwhile to consider if an "AMD style" power supply (lots of +3.3 and +5) or a "P4 style" one (lots of +12) is better.  If you don't have the extra 4-pin power plug on the mainboard, it's likely the "lots of 3.3 and 5" is a better choice)