sorry, 300W is for older cards like the FX series or the 6200 series. even the 6600GT needs at least 350W. 7800GT ( and oc too) would blow up for sure with a 300W. get at least 450W and should have at least 30A on the 12V rail/rails
if you still dont know what that means, look at the sticker of the power supply
it should say a number under or above 12V...if there are two 12V rails...then add those numbers together to see if it is greater than 30 or not.
Intel Core 2 Duo E6420 Conroe 2.13ghz
Gigabyte P35-S3 Motherboard
A-DATA 2x1GB DDR2 800mhz memory
Geforce 8600GT 256MB GDDR3 PCI-Express