At the moment the Seasonic Focus Plus is going really cheap. I'd recommend the 550w version of this.
I don't think you should go below 500w, as anything less will limit system upgrades in the future. Good model PSUs include:
• Corsair CX/CXM - good budget ranges, nothing amazing, but won't blow up either.
• Corsair RMx/TXM - not sure about their differences, but these are the mid/high end Corsair ones.
• EVGA Supernova G1/G2/G3 - All good quality, high efficiency etc.
• SeaSonic S12II - old now but still going strong and frequently recommended.
• Seasonic Focus Plus Gold - high quality and regularly recommended in everything from low end to high end builds.
• Seasonic Focus Plus Platinum - Like the one above, but more efficient.
• Antec EarthWatts Gold Pro - based on the strong Seasonic platform.
There are likely many good ones I have forgotten.
And a few traps to not fall for:
• Corsair VS - like an upgraded day-to-day/office PSU, not good for much more.
• EVGA 80+ Bronze series - same as above.
• Seasonic S12III - would seem like a newer S12II at first glance, but not Seasonic made, and not good quality either.
Other really cheap ones that seem too good to be true when compared to my above 'good' list are also probably not too good either.