While you won't need a 600W monster, you do want to buy a quality PSU that can accomodate for the future. You won't need to spend over $100, but I would recommend that you avoid anything under $50. I'm going by prices at Newegg, so prices elsewhere may vary.
The cheapest unit I can think of is the Fortron (FSP Group) AX450-PN. It's a 450W, but the specs on it could easily pass for a 500-550W unit. It's about $50+ shipping.
If you want to step up, the Enermax EG-565P-VE (535W) or Antec TruePower TPII-550 (550W) run just under $90.
Another solid unit is the Tagan TG-480-U22. It's about $80. Antec's TruePower TPII-480 is also around that price.
If you want a quieter unit, the Seasonic S12's would be another option. Their units are more expensive than the ones I've quoted, but they do provide good power, so anything from them with 450W or more will be fine.
I could go into more detail about the PSU, but the PSU sticky is linked in my sig to it's location at the top of the PSU forum.
