Both T7100 and T5550 is based on Santa Rosa platform (65nm)
As you've said T7100 has 800MHz FSB while T5550 has 667MHz.
However, for all Santa Rosa platform chips, only DDR2-667MHz is supported. 800MHz RAM on a T7100 will be only running 667MHz!
Also if you noticed, Clock speed for T7100 is 1.80GHz vs 1.83GHz for T5550. The difference should be very little.
This due to the frequency stepping.
T7100's 9x 200Mhz vs T5550's 11x 166Mhz
(M0 stepping vs L2 stepping)
Intel Dynamic Front Side Bus Frequency Switching is only supported for M0 stepping I think. This feature will save you power@battery when your CPU is idle. It will throttle down the FSB frequency when idle.
One more difference is Intel's VT (Virtualization Technology) exists in T7100 but not T5550. Basically this feature will be useful if you use Virtual machines. If not it shouldn't be too much of a concern.
My conclusion is that
1) T7100 is better for power saving and if you run VM wares, it will be faster than VM on T5550.
2) If you don't care about CPU idling power savings and VM ware processing overheads, there's negligible difference for computing performance.
edit:
" In newer systems, it is possible to see memory ratios of "4:5" and the like. The memory will run 5/4 times as fast as the FSB in this situation, meaning a 133 MHz bus can run with the memory at 166 MHz. This is often referred to as an 'asynchronous' system. It is important to realize that due to differences in CPU and system architecture, overall system performance can vary in unexpected ways with different FSB-to-memory ratios. "
source:
http://en.wikipedia.org/wiki/Front_side_bus
But since Santa Rosa can support only up tp DDR2-667, a DDR2-800 will most likely be still running on 667MHz