Dont spend any more money on it.
300W...thats like nothing. and im betting the system isnt much if its that size of psu, so upgrading the gpu ram and psu wont do anything if your cpu cant keep up.
I have a 700W and really I wouldnt put anything less than 600 in a gaming system.
Get a nice big 800W with a large fan so its more quiet.
Core 2 Quad Q6600, great bang for buck and a great overclocker.
get an Asus board, something over £80-100.
Grab 2 x 2 GB DDR2 -800, preferably something thats got decent heat sinks since you'll clock it up to 1066mhz. DDR3 at 1066 is slower because of its higher latency
Also get a nvidia GeForce GTX260, and overclock it.
NOBODY (except those who run 2 graphic cards together) needs more than a 550W Power supply! actually, even 450W is enough for most PCs, as long as you stick with good brands such as Be Quiet! or Corsair of course. Getting an 800W PSU only means a higher price and a higher electricity bill since it sucks a little more power in standby-mode