You're not going to be able to upgrade a GPU or CPU with that motherboard, it's just not happening.
What I could muster up for $300:
AMD Phenom II X4 965 Black Edition
ASUS M5A78L-M LX PLUS AM3+
CORSAIR Builder Series CX430 430W
100357-2GL Radeon HD 7750 2GB
Total: $289.96 w/ PSU rebate (Rebate is $10)
You'll probably need to (what I mean is you'll definitely need/have to) upgrade your RAM too if your motherboard is that old.
G.SKILL Ripjaws X Series 4GB (2 x 2GB)
Total: $319.95 w/ PSU rebate
or
G.SKILL Ripjaws X Series 8GB (2 x 4GB)
Total: $339.95 w/ PSU rebate