Use the online "PSU Calculator"
There are a couple if you type it in on a search engine like Google.
Then add 50-100w to be safe and to allow for future upgrades, like a better video card.
Now find one with a 12cm (120mm) fan (or a rarer 14cm fan) and do some reviews on it. Most 120mm fan PSU's are quiet. The cheapest reliable ones might be Antec, a company which many people love. I like Enermax Liberty PSU's. Some people vote against modular cables, others for them. Thermaltake has some new PSU's out and they seem to be cost efficient too. A lot of people like Seasonic as well, which I believe is a British? brand. There are several other good brands too, some of which might be cheaper and several which are more costly. Good luck. Check pricegrabber.com or maybe froogle. I recommend buying new at any rate.