the GF2 Pro is clocked @ 200/400, and the Ultra is clocked @ 250/460
what that means is that hte pro has a core clockspeed of 200, and the Ultra has a core clockspeed of 250
the Pro has 5ns DDr (meaning it runs @ 200 MHz). Due to it transfering twice/cycle, the "effective speed" of the RAM is 400 MHz.
the Ultra has a 250 MHz core, with 230 MHz DDR, effectively 460.
the Ultra outperforms the Pro in higher resolutions @ 32-bit color, i.e. 1024 and over, ESPECIALLY in 32-bit.
Both cards are 64MB DDR cards. In fact, the Ultra and the Pro are the 2 fastest gaming cards on the market, significantly so, in most cases.
the Pro is, IMHO, one of the 2 "best buys" on the market if you want to pick up a card now and hold it until the end of the year, along with the 64MB DDR ATi Radeon.
the Radeon will be able to handle most of the features coming in games this year, but it may get a little slow at higher resolutions due to lack of fillrate. The GTS Pro should be able to handle just about every game that comes at it this year, at decent speeds and resolutions.
I don't see any games coming out this year that will render the GTS Ultra unable to play 1024x768 comfortably.