Also, keep in mind that Second Life uses your CPU more than your GPU. Eventually this will change, but at the moment, that seems to be the case.
I use an HD 5850 at the moment with pretty much maximum detail on a Q6600 but I still wish I could get better fps. My cpu cores are usually hit pretty hard:http://thebotzone.net/tweaks/
There is a new Second Life viewer that came out 3 days ago and it has some graphics improvements. You may want to d/l it:http://thebotzone.net/2010/06/23/test-drive-viewer-2-1-beta-released-today/
They have really been pushing the envelope lately in terms of new viewer releases. They are shooting for every 2 weeks or so. Should be an exciting year for SL.
Little Lost Lindenhttp://thebotzone.net