There are a zillion ways to benchmark your system. A common test is Madonion's 3DMark2000. This just looks so cool, but weighs in at ~20 megs., so unless you have a broadband connection... You can download it at:
Games, esp. Quake III, are another common test (assuming you have the games). There are instructions to run the tests in a lot of places. Although I've not used these specific instructions, this site lists them for many games:
I did try MbM ... but it says my CPU temp is 24-25 celsius I can assure you when I touch it it is more than that ! hehe
If you want to be sure that your CPU isn't too hot, then run a programe that uses 100% of the CPU for a few minutes, then you can put a thermometer on the heatsink of the CPU (on the metal part). It gives quite a good idea of how hot you CPU is. If you want to have it permanently displayed, then go to Reno-Depot and buy a cool digital thermometer and make it fit in a unused 5¼" slot cover.