The difference between a 2GB or 4GB kit is about €30,-.
You're not gonna loose the house over €30,-, right?
And it's cheaper in the long run to get 2x2 than 2x1 and another pair later.
To be honest, I'd be more concerned about that PSU, especially with a dual-GPU card. If money is tight, forget about the memory, just get a decent PSU.
I'm guessing this is some pre-built system? You'd save a lot of money getting the parts and putting it together yourself. 600,- can go a long way, but not if you are paying someone to stick a few components in a case and put in some screws.