video ram in your graphic card is very important too.
i test with sapphir x4870 512Mo, two x4870 512Mo, msi GeForce 285oc 1Go and now i m with sapphir 5870 vapor-x 2Go.

i had i7920 stock + 12 Go ram and display 24in 1900X1200.
OS w7 x64 ssd intel 80Go
wow data syslink on adaptec 2405 raid 0 X4 ocz solid cor 30Go 1er generation so
and 5 wow on the lovely 300Go velociraptor ....

with x4870 512Mo, i used full 12Go ram
with x4870 X2 ... it was the worst !!
with msi 285 1Go, i used near 9Go ram but my cpu was near 100% so limited at stock frequency
the follow didnt break but ips drop in dalaran.

these test were on dalaran city, late in the end of week-end when you have many players

now with 5870 2Go and a little x980 3.33GHz .... but 40% on 5 cors ( number 6 is sleeping with 0% ), i use 8 Go ram and to please me i add 2 other screen for the eyefinity so 1x24in + 2X22in.
main 24in full except shadow 2 dots less

For me, the nice i7 920 2.66GHz could breath near 3GHz oc