320mb and 640mb isn't much of a difference. I would suggest you go with the 320mb since the 640mb is only 1% better then the 320mb and cost a whole lot more. Getting the 320mb you will be able to OC it and out perform the 640mb. And yes, the GTX and the Ultra is much better then the GTS 640mb.
The only 8800's that are 2 different speeds are the GTS's, the GTX's are only 768mb, but as someone else said if you can afford a GTX then go for it, the make to get is either EVGA, XFX there the top two brands