For heating a room you need to dissipate maximum power. CPU temps don't matter, as those are affected only by heatsink design.
A bad heatsink means the chip will heat up more, but that greater temperature differential will still transfer heat to the room. A good heatsink will keep the chip cooler, and will transfer heat to the room beacuse it has a better convection coefficient. Either way, once your chip reaches its steady state temp, the heating of the room will be goverened by the power consumption of the chip. Therefore go for the chip with the greatest power consumption.
An inefficient PSU is a great starting point, and you may also want to find an big CRT monitor since those put out quite a bit of heat as well.
As far as the CPU is concerned:
http://www.asus.com/support/en...chref/amd/amdcbda.aspx
http://www.goodwin.ee/sulo/Power2.htm
It looks like the Early XP Palomino cores would be good contenders, as well as perhaps the Celeron D chips.
I have an XP 1800+ (1.53 GHz Palo) and it had a monster heatsink stock, but now I'm using a Zalman 7000 and it is both cool and quiet.
Good luck with the project!
-D'oh!