I'm running on a pretty old patchwork Dell box at the moment. I've got 1gb ram, a 2ghz CPU, and an ATI X1650 video card. I'm able to run on balanced in low population/lag zones. In groups, harbors or high traffic areas I have to drop it to high or extreme performance or it locks up and/or boots me to character select.
It's playable most times, but now that I've reached the upper tiers and folks have such flashy spells & all the swarm pets I've found grouping to be impossible without locking up every 20 mins or so. In the lower levels I didn't have this problem... my theory being there was less "screen clutter" in the form of swarm pets, swarm mobs, flashy effects, etc. Basically the ram chokes and the CPU locks out at 100%. /shrug
If you're rig fits at least that setup you can play, just bear in mind the more crowded a zone is the more likely you're going to have to deal with a bit of frustration. A bit more RAM would like fix a LOT of that if you're able to get more than 1gb. (My card is maxed unfortunately)