I have a problem with fps while playing wow in a raiding environment... i have a dual core 3.4Ghz pentium , 3Gb DDR3 RAM and Nvidia GTS450 video card which allow me to play at mid- high video settings keeping 50+ fps in stormwind. My latency is also quite low as i rarely see it reaching 60ms.
Though as soon as a 10 , 25 or even 5 man fight (boss or even simple trash) starts fps drops down and become extremely discontinous , and i get huge lag spikes that freeze the game for some secs.
Another problem (maybe not related to this one) is that my pc reacts terribly bad to some video effects : for example during erudax (grim batol) ability gale of shadows my fps doesnt go past 6-7 , same happens during anraphet(HoO) aoe and similar effects.

get a new graphics card