X51 GPU and PSU upgrade

With the release of the AC Unity PC specs I realised that now is probably a good time to upgrade my system. So I've done some research on what I can upgrade but I'm still not sure what will work with my system. I bought my X51 in may 2012 and it has an i7-2600 CPU, a GTX 555 GPU, 8GB RAM and a 330W PSU. Can anyone give me an idea of what GPU's I can use and whether or not its possible to upgrade my PSU. Money is not too much of an issue either.

EDIT: Added tags