It depends on the case. The most common PSU size is Standard ATX, which is in 95% of all cases. There are many others such as
CPX - An antec PSU size which is a third larger than ATX and fits a few of their Performance One series cases
SFX - A Silverstone PSU size which fits in a small amount of their cases but does comes with an ATX adapter as standard
Micro-ATX - A smaller and longer version of ATX but fits in most ITX and HTPC cases
The companies I would go for a PSU are XFX, SeaSonic, Antec, OCZ and Corsair. All are excellent quality and deliver good performance.
Companies I would stay away from are CiT, Ace, EZCool, Sumvision or any cheap power supply make as they have false power supply ratings, poor quality and give fewer cables than any good make.
But if you're in to overclocking and you don't want your power supply to limit it then you should buy this one instead.It's less power efficient which might account to a power bill as much as 15 cents bigger a day.It also doesn't use a modular design.But in reality it can provide almost 1000W of power so... yeah.
Seasonic and Corsair (TX series) and PC Power and Cooling make excellent PSUs.
There are a few others, But Highly recommend that you check for reviews, just google model - NO review - NO BUYee. You can get some idea by looking at the reviews on newegg, take with a grain of salt and only look at if Nr of reviews is say higher than 100.
^ Reason PSUs are concervative is that they try to take into acount the "POOR" quality PSUs that many foolishly buy.
"little extra voltage " I know you really meant little extra Current cabability. They should never increase Voltage.