After a little more thought, I'm inclined to agree with Invisik.
But, do not look for the GDDR4 version of the HD4670, that would, I think be just that little bit too much.
A look on the Gateway site shows the PSU as looking to be standard ATX format, so a replacement should be easy enough.
But what do you want to do with it? Even the HD4670 is quite weak for gaming, and, frankly far too much for even HD replay.