Well, I don't think that's really the entirety of the issue. You need to consider what you'll be upgrading to. The overall system runs best when the FSB of the processor and the memory bus are syncronous. You really don't gain much, if anything, from running the memory at speeds faster than the processor in an nForce2 board. If you'll be running one of the new Athlons at 333 MHz (166 DDR), then PC2700 is all you need. However, if you're the type to upgrade fairly regularly and you think you'll be going for the 400 MHz Bartons (200 DDR) when they come out, then having PC3200 ready for it wouldn't be bad at all. Alternatively, if you like to overclock and want to get your processor's FSB to 400 MHz or possibly more, having PC3200 would ensure that the memory won't have issues.
Don't waste the extra money on something that you won't benefit from. Basically what it boils down to is: get PC3200 if you're planning on getting a Barton or overclocking. Otherwise, there's not much to be gained.