While, untimately, PC3200 is going to be faster than PC2700 at maximum speed, the Athlon64 architecture is not drastically affected by memory speeds. Yes, there is a hit, but it's the type of hit that is only detectable with benchmarking, and the performance hit is so minor, you simply won't notice it.
The Athlon64 has the memory controller on-die, unlike traditional CPU-Chipset-RAM designs that have the memory controller as part of the northbridge. To operate with the RAM (as we don't have 2GHz RAM modules around---well, at least not that you can buy), there's a divider in the system already, which negates a lot of the performance hit you'd typically see running slower RAM than what you should pair with a traditional setup.
Applications care much more about the amount of RAM than the speed. Now, I wouldn't get PC2700, simply because PC2700 is likely to be the same price or more expensive than PC3200. Since the PCxxxx ratings are just the maximums, you shouldn't run into any speed conflicts in your system, as the BIOS will relegate the speed to the lowest common denominator by default.
