I have a HDD I can use
Good, I just added this up on Newegg:
8GB Corsair XMS3
Sapphire HD 6850 1GB
Actually comes closer to $600 but I don't know where I would try to save money there to be honest, depends which is more important out of gaming and work.
If gaming is more important:
Change to i3-2100 and 1 x 4GB RAM
If work is more important:
Change to Corsair CX-430 and HD 6770 1GB
Maybe get a slightly cheaper case or motherboard too, reusing an old DVD burner would help as well.