Actually, I think Prime is a VERY good test of stability and a stable rig should pass both Prime AND 3DMark, as well as memtest86 and Sandra, and if it floats your gravy, PCMark (though as a benchmarking utility, i think its a lil off).
I personally think that running two burn-in/stability test at the same time after passing each one individually is a great test of stability. After you pass Prime and after you pass 3DMark, run em both together.
No offense directed at you, Hangten, but there are several folks on the board who tout "my system is stable, but I can't pass Prime - so Prime must be a crappy-program/faulty-program". Well, you can play games and browse the net on systems that don't pass Prime but a measure of stability is just that - the rig is able to handle a big stress/load on the various components and handle the load. Some games are CPU intensive, other memory, and others still vid card. Very few, mostly like no games, stress all the differnent componenets at the same time. Thats why we run the said testing/stressing programs to ensure stability in all factors of our system.
3DMark is a great proggy to test vid card stability, as it heaviliy stresses vid card, but I would recommend that other factors/components be included in the stressing to ensure a nice, well-rounded stable system.