You should be able to throw a 7900 GS Go in there. Unfortunately,
Dell parts doesn't list it, maybe it is to new (or to popular) of a part. Since they still sell this part in their notebooks, they obviously have them, so I'd give them a call and see if they will sell you one (1-800-357-3355). Fair warning, it won't be cheap. In fact, I think that even if they would sell you one, the price will eat up the $$ you saved buying a used 1705, and you'd end up better off to return the ebay one and buy a new one with a 7900 GS.
If you do end up buying a 7900 GS Go,
here's how to install it.