Most PSU will tell you what MOBO they will work with i.e. ATX 24pin, etc. A lot of a PSU purchases depends on what else you want to run on/in your system. Why dont you tell us what mother board it is and we can help you out.
ASUS has poor reliability. What you system specs, with out knowing everything your running its hard to judge how much you need.
phenom II x4 3.4ghz-cpu biostar mcp6p-mb azza orion 202-case hp 24x sata-dvd burner 4gb 240pin ddr2 800(pc2 6400) 1 tb western digital hard drive am3 heatsink and fan 500 watt power supply geforcce 9500gt-vidoe card
Whats the make and model on your psu? If its a quality psu you should be fine where you are depending on what future upgrade you have in mind. Unless your current psu is dead or something.
if the psu calculator says 378 watts is a 500 watt psu to much or enough and i want to get a APEVIA ATX-CW500WP4 500W ATX Power Supply
PSU Calculators are pretty much always worthless. However, you dont have to worry about delivering to much power. Your PSU, if its good, will take care of how much power is or isnt needed. However, you dont want to go to low. Doing so will starve your components and cause problems. You could throw a 1000 watt psu on there and be safe, but the psu you have, to put it bluntly is worthless. If its super cheap, dont but it, atleast in terms os PSUs. When you pick it up does it feel pretty light? If so that another indication its worthless. A quality PSU will be noticeably heavier then a cheap junk psu. Anyways, I would get a new psu no matter what. Go for a 500 watt corsair. http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004
