My mobo is an Asus P5N-E 650i, and because it is very finnicky with the memory, I have been using manual timings with the 2GB, set at 4-4-4-12 22 2T. I put the other 2GB in for a total of 4, and so far it is working fine. Will I see a performance hit by doing this? Should I underclock to 5-5-5-12, or keep it at 4-4-4-12 as long as there are no stability issues? One set of RAM is 1.9V and one is 2.1V. Don't know if this makes a difference either. Thanks!
They seem pretty similar, possibly the same modules, with the only difference being the CAS4 sticks rated at a higher voltage to achieve CAS4. Simply set all the sticks to run at 2.1V and you will probably be able to run the CAS5 sticks at 4 stably. If problems occur, just take the small performance hit, like 5% worst case, and run all at CAS5.
2.1 is the maximum voltage. It really isnt set in stone, as long as your system is stable you're golden. Just set the new pair to 2.1v with 4-4-4-12 timings. It should be stable, but if you run into issues put em to 5-5-5-12. Its not a huge performance loss, and you're likely not to notice.