the default install of SuSE 7.1 has a bootmenu option to do memory tests. I don't know what pkg it runs to make it go... could do some research I s'pose. This one helped me diag a beauty SCSI server mobo w/ dual PIIs and major onboard monitoring. Bad RAM (HP OEM - found out later it was a recal item) from a swap meet was the cause. Real RAM will stil set me back big bucks.
Putting 'bonnie' in a loop of a script would be a nasty workout for I/O. (I did it to test a RAID array).
Anything that will work out the mainboard chipset, even including some hardcore grafiks.
kfract (fractal gen) made my power supply hum over extended runs. That got replaced real quick.
Hate to admit it but Intel chipsetz ('cept for the frikkin Camino/i820) has been pretty good to me.
Make it go and make it go hard.