Actually it can cause poorer performance or better depending on the application.
The CPU doesn't need the extra bandwidth, but if you have a single channel setup with PC2700, and use a game that causes your video card to use AGP texturing (like if you have a video card with very little onboard memory), then the extra bandwidth helps because the AGP card accessing main memory isn't taking away bandwidth from the CPU to the memory. But with a 32MB or larger memory size card, AGP usually isn't even used much.
However, when you're running the memory at a different speed than the CPU bus, there are latency penalties induced because a cycle of the memory bus takes less time than a cycle of the CPU bus. So sometimes when the CPU needs data, the memory controller has to wait part of a cycle for the memory to "come around" for the next cycle because the memory may have already been in the midst of a cycle which can't be interrupted. Sort of like having a bus line that sends a bus around every 10 minutes, but people line up at the bus stop every 8 minutes. Sometimes the bus will arrive at the same time as the people, but most of the time the people will be waiting around for the bus, or the bus will come by and not have any people to stop for. The bus can carry the people just fine and have room to spare, but the timing isn't lined up.
With dual-channel, even running at 266MHz, you've got double the bandwidth that the CPU needs, so the AGP card isn't going to make the CPU run out of memory bandwidth if it did use AGP texturing. So there's no NEED to go any higher, and you don't run into any latency penalties. This is different from CAS latency, which is entirely the memory bus's timing; I'm talking about latencies and delays between the two busses. Using 2-2-2 memory at 333MHz can actually be slightly slower than 2-2-2 memory at 266MHz. The difference may or may not be huge depending on what's being done.
Also, with dual channel on the nforce2, the chipset uses two memory controllers to "hide" the inherent latencies, by queuing up memory accesses ahead of time, so that as soon as one access is done, the next one is ready to go. This helps to also increase performance over single channel.
majewski9 recommends getting PC2700 or even PC3200 memory now in case you upgrade and need to run a faster bus. This is perfectly valid, and you can still run the faster memory at a lower speed to get the synchronous timing. The point is you have to decide whether it's worth paying extra for faster memory that you can't make use of right now but may or may not save you money in the long run. If you decide to upgrade, you could use the same memory, but what if you also make the old machine into a second computer? PC3200 memory will probably be cheaper by the time you upgrade, so if you buy it now you've basically paid more than you need to (in my opinion).
Depending on the brand and ratings of the memory, PC2700 isn't all that much more expensive than PC2100 (from Crucial, it's only 4 bucks more at 43 dollars), whereas PC3200 can be a LOT more expensive, especially for the low latency rated modules. Crucial's PC3200 256MB is 79 dollars, and that's CAS3 memory, not even CAS2.5 rated. To me, PC2700 is a good buy because it gives you some future upgrade potential, and doesn't cost much more, and you can run it at a lower speed with good latency settings (Crucial's CAS2.5 PC2700 should work well with mildly aggressive settings at 266MHz). PC3200 is a waste of money if you can't use it anytime soon.