To get a good idea of your needs I suggest you use this: http://extreme.outervision.com/psucalculatorlite.jsp
you just fill in all the info about your PC and it should calculate your total need of wattage. To find out if the new PSU will fit in the chassis of the case you have to know what type of tower it is, usually its type ATX if its a mid-tower case which is usually mentioned on the PSU, motherboard, and the case. If it is ATX I would suggest this PSU for your needs: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371029&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction&AID=10440897&PID=3067296&SID=
but take note that it doesn't come with a power cord. But if ATX is not your case's type I suggest to generally look into the Antec and Corsair brands, both are very reliable and wont let you down. I hope this helped you.