The 366@550 would run hotter than a 866Mhz
Although that depends on
A: heatsinks
B:What the 866Mhz is e.g. Slot1 or Socket 370.
If its a Slot 1 then it will be about the same temp but since there is two 366's@550 they will tend to run hotter.
If its a socket 370 P3 then it will cooler than the 366's@550.
Uniprocessor P3 866 vs. Dual Celly 366@550 ???
with good heatsinks and thermal paste anything will run cooler 

!!! WARNING !!!
The following forums: www.bp6.com
are infected with the following VIRUS(s): Kuun.infected.all.posts.Win2K.user
The following IRC servers has been exploited: irc.bp6.com
with the Following Exploit: Kuun.lurks.using.mIRC.v5.82.exploit
The following forums: www.bp6.com
are infected with the following VIRUS(s): Kuun.infected.all.posts.Win2K.user
The following IRC servers has been exploited: irc.bp6.com
with the Following Exploit: Kuun.lurks.using.mIRC.v5.82.exploit