Basically with an XP rig, if you run your ram at a different speed than your cpu by using the memory dividers you take a big performance hit, therefore, you should always strive to be running at 1:1 memory ratios. The memory ratios, by the way, will result in a constant
relationship between your ram and cpu, not a fixed "XXX" speed for the ram regardless of how much you raise the cpu's speed. The ram speed would still increase when using the memory ratios, just not at a 1 to 1 relationship.
With that in mind, you are left with either upping the multiplier for an overclock or keeping the multi the same (or even lowering it) and upping the FSB. An XP rig running at any given speed will perform much better with a lower multi and a higher fsb than the alternative option of a lower fsb and a higher multi...220 x10 is better than 200 x 11, even though under both alternatives the cpu speed is the same 2.2GHz).
Working within that basic premise, the ideal approach would be to gradually increase your fsb, possibly requiring you to relax your memory timings and/or increase voltage along the way to remain stable (voltage increases may be required of the ram, the cpu and the northbridge, as well). The only way to know the optimimum settings for your particular system would be through testing with benchmarks as you go through your tweaking until you find the combination that yields the best benchmark results (SuperPi, 3DMarks series, etc) which should translate to better real world performance.
I have always found the following articles to be very informative.
Memory Basics -
http://forums.pcper.com/showthread.php?t=320351
Overclocking XPs -
http://www.neoseeker.com/Articles/Hardware/Guides/OCguide/
Good luck and I hope some of this helps.