I'd probably go for about a ~500W PS. You can go a bit bigger if wanted, but a 750W would likely be overkill unless you are running something like dual SLI video cards. Not much advice but to stick with a well known brand, Seasonic, Thermaltake, Antec, OCZ or similar. Make sure it has the proper connections for your motherboard and video card. I like the modular supplies as you can get rid of some unneeded cables and get better airflow most times.
Most common PSs use the same size, bolt pattern, etc,. so no real problem if you have a standard case. Some proprietary cases from Dell or some others use a special size PS, but that's not common.
If you want a quiet PS, then ones with larger fans are often a good choice. Most of my Thermaltake PSs use a bottom air intake with a 140mm fan and they run very quiet. Bottom intake PSs also act as a case fan and may help with exhausting warm air from the case.