PSU's are a standard size unless you have a Dell or a shuttle as they have different size PSU's. You can have any wattage however a big one is better as will run more things and so better for upgrading.
One thing to note is some generic PSU have a rating of 400w however this may only be a MAX output and its really say a 250w.
If you have the money go for a well known brand such as ANTEC, ENERMAX, TAGAN & OCZ.
Expect to pay a decent amount for a good PSU it may seem alot now but then think how much it would cost to replace your whole pc and data if it fries your PC. 330w - £35~45 430 - £55~65