It's not a question that the 7600 doesn't need an external PCI-e connector. It's a question of total power draw and amperage required. Those are constants, whether it's a 7600GT or an 8800GTX......same rules still apply.
So the fact that the 7600 draws power straight through the mobo is irrelevant. He needs to verify that his PSU is able to power the card. 300W is cutting it
extremely close. I certainly wouldn't feel comfortable running it on 300W, and certainly not without verifying the amps being provided.
Might run, might not. Might run for awhile, then crap out his system. Personally, I wouldn't take the chance. Some might, but I'd rather spend $40 on a PSU than blow a system
As for installing a PSU Thomas, it's quite easy. Unplug the power cord from the back of your tower, then unscrew the PSU at the back, then remove it. Put in the new one in reverse order. THEN, so you don't get lost with all those connections, just unplug one at a time from the old one, and put in a new connection that matches, one at a time. It'll be easier for you to follow what connector goes to which device. It's just like matching pairs.....remove the old cable and plug in a new one that looks exactly the same. Piece of cake.
Plug back in the cord, turn on the machine, and there ya go!