As a test I loaded all clients up on DX11, and saw no difference really.
I've since upped every graphic setting in the game to Ultra High/Very High, and ticked every box I could find (except for Vsync).
This make my GPU run at about 30%, 3GB vram, with overspill into system ram of about 1.5GB. The main client runs between 30-50FPS. I would probably do better with a 4GB card here, but the CPU isn't too busy it can't cope, and there is plenty of system ram left.
This is only running around in the environment. A big battle would probably kill my FPS.
BTW I'm using a 3GB 7970 driving 6 2560x1200 lotro on one screen, and a couple of other screens I use for other stuff, like this forum
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edit:
It seems that you don't really need triple buffering under the Adv Graphics. This just takes more vRAM. And the Antialiasing under std Graphics settings is what causes me to bog down. I can run all 6 clients @ 2x without issue, and still get decent 30+ FPS on the main, but it can get boggy when going higher. All depends on how fancy you want it to look. I've played for years with no antialiasing and fairly low settings so at the moment everything looks super pretty