Sorry for the semi-sarcastic reply.
It's unlikely you'll see any difference at all.
Since they're rated at the same speeds (assuming they have the same timings), they'll likely performe the same. XMS is designed for overclocking.
In fact, unless you have a 1333 FSB processor (most likely 1066), you'll be just as well off getting DDR2-533 if you're not overclocking. If you were to get DDR2-667, to use the RAM at 1333 and the FSB at 1066, you'd have to run the RAM async, which usually involves taking a performance hit instead of a performance gain that would might expect from the faster RAM speed (because it's async).
So in short, if you're getting a 1066FSB processor, you can get DDR2-533 and be just as fast as the DDR2-667. The faster speed means it CAN run faster, but you generally don't want to unless you're overclocking. Also, getting DDR2-533 means that your RAM will probably have tighter (i.e. lower) timings, which might get you a FPS or 2 over the DDR2-667.
If you're not familiar with RAM timings, getting DDR2 at 4-4-4-12 would be better than getting DDR2 at 5-5-5-15. Generally, lower timings are always better, but faster bus speeds are not always better (unless you're overclocking).