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.
In real world terms, no. Its cheaper, faster and easier just to upgrade to a better card, but the extra memory wont really see you gaining much unless you run the car with a 30" monitor maxxed out. Just stick wit the 320Mb version for now, and enjoy it.
I agree with prozac and Whizzard. It's not a question of how much of a hassle it would be. It's impossible. Period. You have no way of knowing how Nvidia routes communication between the RAM and the GPU, regardless of your soldering skills. In all likelihood the chips are machined on by, uh...machine. Therefore, doing so by hand would be impossible as you could never emulate that precision. This process would be tantamount to changing a 1 GB DDR2 stick into 2 GB by soldering off its chips and soldering on new higher capacity ones. It just don't work, suh. :wink: