Need help upgrading my system

hey,

i got this computer I build it back in 2007 (too old i know) but cannot afford to buy a new system because if I want to i would buy a gaming pc atleast 1000$ build so i wont regret it

anyway my pc specs:
intel pentium 4 631 3.00GHz HT cpu
Foxconn 45gmx series motherboard
xfx nvidia geforce 8500gt 512mb gpu
2 kingstone ram total 3GB ddr2
3 H.D.D's total 500GB
intex 400w psu
and a cheap etek atx case

now i want to upgrade the cpu and the gpu to this
intel core 2 duo probably E6600 or higher
amd radeon hd 6670 (yes my motherboard can run pci 2.1 cards on it's pci 1.1 slot but less performance of memory)

what i know that pentium 4 draws more power than core 2 duo i guess more 20~30 watts
but the radeon hd 6670 draws more power than geforce 8500gt about 18 watts more

anyway i dont know what to upgrade first the cpu or the gpu?
because i can afford one of them now and the other later
so for gaming what is the best choise?
thanks.