Build for my friend, need a little help... $400 - $500 budget

[Zerokateo]

Alright so my friend wants to upgrade his build a bit, he currently has a dell XPS 630 which has a q9550 core 2 quad core processor, 8 gigs of ddr2 and a 9800 GT. He wants to upgrade because this is starting to lack in games. Now heres the problem, he has about $400 - $500 to spend upgrading it but he wants to use the dell case and the PSU in it which is 750 watts, the XPS 630 uses real ATX motherboards so hes fine there but I have no clue on the quality of the power supply and if its trash then that means his budget is too small for any 'upgrade'.

I've picked out an asus z77 and an i5 2500k with 8 gigs of ram but the total of that comes to $473.97, so IF he needs a new PSU will the point of where I need help picking something out for him.