I have an HPa6300f pavillion, I'm going to upgrade to 4g ram, GT9800 512mb video card. I need a bigger Power Supply, probably at least 600w. How do I KNOW what size to get so it will fit in my case? Any reccomonendations or suggestions. I want to keep it under 100 dollars if at all possible.Power Supply's Are pretty much universal in size for the most part. Make sure when purchasing the power supply it comes with the proper connectors for your PC's components. I purchase all my hardware at Newegg.com / tigerdirect.com Here are a few PSU's I recommend:

HTTP://www.newegg.com/Product/Product.aspx?Item=N82E16817341001

http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004

http://www.newegg.com/Product/Product.aspx?Item=N82E16817371015

And i do believe all the products i just LISTED have FREE shipping! Mini ATX is a bit smaller than regular ATX but if this is a Pavilion it will take an ATX size...
As stated decide on a price point and post your specs of what else is in the machine and we can advise further....http://www.newegg.com/Product/Product.aspx?Item=N82E16883107511 here are my specs. Okay that makes sense, so ATX is a size of power supply.Correct...