Under blend mode Prime95 overnight should bring an indication to stability.
If you want Prime95 to push harder set the memory size higher.
When the 'Run a torture test' window pops up click custom and increase memory to use(in MB)
Default is 1600MB. Set it to use most of your ram, keeping at least a gig free for the rest of the system, not only that if you set it too high the operating system will start using page file and memory wont be tested properly. Leave 'Run FFTs in-place' unchecked as this is more to do with cpu.
Running Prime95 at default 1600MB however will end with the same results, if theres a problem will find it, just slower is all.
Another popular test you can do which is arguably better than prime95 at testing ram is http://www.memtest86.com/