No problem!
About memtest...I think it would be much more conclusive if you run it. If you run it and it starts throwing a bunch of errors, then it's pretty easy to say the RAM is at fault. Especially since there is another question mark in the problem (the second graphics card). I should mention there is a version for a USB thumb drive too...so if your motherboard can boot from a USB device, you should be able to run it no problem ;)
Also, to answer your question about the second graphics card...it
could cause stability issues as it will stress your PSU a bit more. So, if your RAM tests OK, that's the next thing you should look into.
And don't worry about the warranty on the RAM...G.Skill has a lifetime warranty. If your RAM is indeed pooched, just contact them and they should help you out getting it replaced.