Try settings from here
https://github.com/litecoin-project/litecoin/wiki/Mining-hardware-comparison#wiki-ATI_AMD_GPUsProcessor doesn't matter in this type of situation, I have I5 3570k on idle while mining and two 7950. One giving out 730khs and other 620khs (could be 700+ if they weren't so close in my case)
To get better hashrate speed you have to spend some or a lot of time trying to overclock and tweak settings, but anyways check out that it should give you enough.
Intensity idk really (newbie) but I think it's for how much % usage you're letting the program to use your card. For 7970 I think for some reason the best intensity is 13.
No, PCI-E 3.0 doesn't make any difference, in fact, some people are running their GPUs from just PCI-E x1 with special risers/exension cables (x1 to x16)