yeah for your system you should be ok running until you hit northrend.

pre-bc i was running 5 accts on my rig; core 2 duo 2.6, 9800gt 512mb, 2 gig ram, sata 2 drives

its doable, but the addon cities kill performance, everything on low, i dont even go to dalaran now, i've since dropped the 5 accounts down to 2, since i had to buy a car since my last one died. even with 2, its horrid there. quad core and up need apply heh. anyways good luck