Don't think it is even possible to upgrade:
You need a new CPU, which will not fit on your old MB, so you get a new MB + GC, then you will need a more powerfull PSU because your old one most likely won't have enough juice for the new GC and CPU.
RAM: you'll go for DDR3..your old RAM must go.
So realistically speaking with 1000€ you're good to go, sure you can still save here and there but not much..