I got a Dell inspiron with the same GPU and i've looked at the options... (not exactly replacement, but for a more powerful gpu)
Basically the only GPU that you could replace it with is another go 7900 gs or a go 7900 GTX.
a go 7900 gs right now runs about $75 on ebay. while a go 7900 gtx is around $90
replacing it is also a pain in the ass since you literally need to strip it down and even file off some of the casing (to fit the bigger heatsinks in for the go 7900 GTX)
imho, you'd be better off getting a new laptop.......