I like how the games you mentioned are very very CPU heavy, not GPU. GTA is a terrible terrible port :/ only 1% of the computers w/ steam have 3 cores dumb ass coders.
Anyways are you talking about the 9800GTX? you just said GT the GTX takes massive power to run and it does match the power of say the 260GTX but there are better cards for the money and power usage.
Perry it may say max 300Watts but that means nothing about the quality of the PSU, I would never try to got to the "max" of a prefab PSU most of that wattage are on the 3.3v and 5v not the 12v where it is needed