300watts is 25amps. Thats not too shabby. And at only 2 years old its probably still has most of its rated power.
Unless you have lots of case fans, hard drives and optical drives it should be enough.
In this example system with an 8800GS used 221watts under full load with an overclocked Q6600 CPU.http://www.tweaktown.com/reviews/1370/18/page_18_power_consumption_tests/index.html
If that type of info isnt enough for you - try hanging out in the Sony support forums and see what others with the same model PC are doing for upgrades.