I'd use the 1Ghz for now, but as temps rise and in the middle of summer I might be tempted to drop them down further.
I keep a few different profiles saved on MSI afterburner, a under volted stock one for lowest power idling I can get away with, a "this game is so old excess speed is just a waste" pretty much stock clocks setting, though my 5850 will undervolt at stock fine. Then I have a pretty standard 925/1.2v overclock, and a silly "if its cold or I'm playing Crysis/Metro" type overclock with a little more juice and clocks.
When I play lotro, i played windowed so I can watch tv, browse the net, whatever, I play vsynced most of the time aswell, at 60fps the gpu usage is barely 50% on max settings, no point overclocking and using more power, when the card isn't even fully loaded.