If you just up your multi on the CPU without upping the FSB you are only increasing the speed of your CPU and basically creating a bottleneck effect.
Well, not really. AMD's will gain more from a higher clock speed/FSB ratio, as the mem controllers are internal to the cpu. So memory bandwidth is increased as the cpu is. Latency is by far, something that affects AMD rigs more than Intel too, lowering the multi on the CPU will add more latency. Im sure you can reach a point in the multi/FSB where you are able to overcome the added latency with raw speed, but all in all, I would stick with the higher multi for a more responsive CPU.
Q-> If your RAM can run at 450mhz...then why not just leave the multi alone and up the FSB until you reach that RAM speed? As long as your cooling is up to par, you should get 'the best of both worlds'...faster cpu and mem, without extra latency.