I agree with the RAM. I started with 4 as well and once I went into wotlk content while leveling I constantly lost follow, had screen freezes. Even with details off on all chars. I upgraded to 8 and the game ran great with 4 chars low details and 1 nearly full(shadows 50% but runs with full).

It`s funny because before that I ran 4 shamans on a singlecore2800+ (if I recall correct lol) with 2gig ddr1 266hz. I had all details low and view distance on all at around 50% and it ran better then 4gig ddr2 and a quadcore3gig. Pretty weird haha but anyway 8gig is the way to go, I use around 5,6gig of it in dalaran.