The GX7 didn't have PCI-express on them, so they can not be swapped out as easily as the newer models (the GX7 is from 2.5 years ago). I do think there was a way that they could be changed, but I am not sure if this required a newer motherboard or being sent back to the manufacturer. It has been quite some time since i had the GX7, so I haven't checked it out recently.
Two recommendations for you. First, I would check with Hypersonic and see what they offer. If not, check with some of the other brands that allow you to piece a system (some have posted URLs - but I can't remember one right now). They may be of help as well.
That being said - the 9700 was a good card. It may not be as modern as the newer cards, but I would assume that you would have to replace the motherboard, power and possibly cooling to add the newer cards - might as well buy a new system.
That being said - enjoy the rig.