2GB is pretty much the sweet spot right now.
It seems to be alot better in most games.
Here are some benchmark comparisons: http://anandtech.com/bench/Product/548?vs=540
Thing is i have 200euros to spend on gpu and i thought that this could be the best one for that price.
Is it gonna be ok with 700w psu and 900p?
Yes, this one is good for that price. You could also check out the gtx 660, It seems to be slightly cheaper and only uses one 6pin rather than two. They have very similiar performance, depends really on wether you want AMD or Nvidia.
The PSU should be more than enough.
And I don't see any problems with 900p.
Good luck with your PC upgrade.