The C2D T5500: 1.66GHz, 667MHz, 2MB
or the C2D T5300: 1.73GHz, 533MHz, 2MB?

The T5500 machine is $200 more. Does the FSB really make that much of a difference?

Although the Core 2 Duo T5300 @ 1.73GHz (13x133) is clocked faster in terms of overall speed due to a higher multiplier. In reality, the Core 2 Duo T5500 @ 1.66GHz (10x166) is more efficient because of its higher FSB & it definitely outperforms the 1.73GHz despite being rated at a lower speed.

What will determine if it is worth $200 more depends on what you use the system for at those times when the CPU is not idling -- which is what most mainstream CPUs do 90% of the time.