Firstly, you have to ask yourself if 40mhz (and the inherent joy of rounder numbers) is worth the extra outlay for an SI-120. Admittedly, it will in most cases be quiter aswell so that may be an extra justification. ou won't notice 40Mhz extra. Especially on a 2.66Ghz dual core.
2-3degrees under load might mean that you feel happier giving the CPU a bit more voltage, which will be needed if you want over 2.7 (I should think, unless you have Jesus' own 4400+)
It's up to you. Congratulations on the rig though, any pics? The SI-120 might also help cool some other components around the CPU, which could help your OC/stability. If you decide a 2-3 degree difference is worth it, then make sure you have some utterly invisible wiring and awesome case airflow too, as that can be accomplished for free and could easily help drop temps by those kind of levels.