Anything's possible. But you would have to drop some serious cash to get the proper ram chips for the upgrade. Which ever IC company Nvidia is dealing with to get the ram isnt going to sell just one set of chips, the minimium order is usually a 1000. Also, the 8800gts uses BGA (ball grid array) memory, so somebody with the proper tools and knowledge would have to perform the upgrade. Cell phones use a bunch of BGA chips, and some cell phone repair places will reball chips, so having one swap one set of IC's for another would prolly be do-able. For a price.
In the end it's more trouble then it's worth, since you would end up paying thousands of dollars to get the hardware and have a guy "install" the new ram. I mean, if you're going to spend that kind of cash, you should just buy an 8800 Ultra, it's only a couple bucks more.