To monitor the temperature I use a low cost temperature module from Maplin electronics, with the sensor probe strapped to the lowest part of the heatsink I can reach - the LCD display is then mounted in a spare 5.25 slot cover so that I can keep an eye on things. The module also has an alarm feature whereby a maximum temperature setting can be set and a sounder will activate if the measured temperature exceeds this threshold. I have used a 'standard' high performance heatsink and fan combination without any problems to date. There are a number of people who have experimented with exotic Peltier and even water cooled systems but these are for serious overclocking situations, so long as you are going one or two steps up in processor performance then they should not be necessary. The files were still available on the Taiwan PCChips website as of late October 1998.
Benchmarks show an improvement too.
I'm certainly impressed with the sense of speed in the machine, this is probably helped by the fact that I've got a Creative Labs Banshee video card in here as well.