DON'T: Buy a cheap PSU
DO: Buy a quality unit, such as the XFX 550W
I bought this power supply to replace a CiT 500W that fizzled out on me. I chose the XFX 550W after finding a benchmark test of 43 different PSU. This unit was remarked as having one of the steadiest voltage levels (even when tested at max power output) at an affordable price. It is also 80+ bronze certificated efficiency and had the 2 x 6pin PCI leads I needed for my HD6950 graphics card.
My one gripe is the sheer amount of cabling. Seriously, it reminds me of Medusa's head. Also its non-modular, so you have to struggle to tuck them out the way to maintain airflow through the case. Not an easy task with mATX form factor. Since, I only have 1 graphics card, 1 hard disk and 1 optical drive, 80% of the cables are unused. The cables are well finished, being bunched in black braiding. The downside is that it makes the cables even stiffer and harder to hide away.
After researching, I'd recommend spending no less than £35 (IMO). An extra £20 spent now will give you peace of mind and could save you hundreds. Buy an 80+ certificated unit from a reputable brand, with high Amps (>20A) on the 12v rail(s). Single rail should be seen as an advantage when using multiple high-power components.