This is what I did on my Gentoo 1.4 rc 2 to use lm_sensors in gkrellm.
1) recompile the kernel with i2c support (Character devices -> i2c support -> .. build all that stuff as modules). For RH 8.0 you don't need to recompile the kernel since i2c support is already build as module. No idea for Slackware.
2) make a directory called 'sysconfig' in /etc -> 'mkdir /etc/sysconfig'. This directory is already there for RH.
3) run 'sensors-detect' and press enter to every questions.
4) load the modules : '/etc/init.d/lm_sensors start'. On RH that must be 'service lm_sensors start' .. or is that for MDK ?
5) Start it at boot time : 'rc-update add lm_sensors default'. I think that is put in /etc/rc3.d and /etc/rc5.d by default on RH.. not sure.
6) OPen gkrellm and configure the plugin now. Or in a console type 'sensors'