You know, this topic of what is the best rig for Second Life is rather complex. I dedicated a small forum to it at The Bot Zone in the Tweaks section.
The reason is because at present, SL doesn't use as much GPU performance as it could. Instead, it relies very heavily on your CPU. So if you have a decent quad-core (anything better than a Q6600), and a decent GPU (anything better than an ATI HD3870), you should be ok.
As you can see from this snapshot, my Q6600 is pegged, and this is with hardly any activity going on inworld:
I upgraded from an ATI HD3870 to an HD5850. I only noticed an increase in frames of maybe only 2 or 3 fps. My Q6600 is still the bottleneck. I really wish I could get one of the new Hexacore systems, but at $1K a pop, it's a bit much, and I'm not ready to try AMD's solution. I'm fine with their GPU's but not ready to go back to their CPU's.
Also note, I usually have the graphics settings at close to maximum including 8x anti-aliasing (goes to 16x) and anisotropic filtering.
Anon, did you try both SL viewers? 1.23 and Veiwer 2.1 Beta that was just released the other day? It has some graphics improvements. Which viewer did you have incompatibilities with? The latest Viewer 2.1 Beta refined a few graphic items including additional driver support, and by looking at the date of your post, you wouldn't have been able to try this latest viewer as it is only 3 days old and you posted 6 days ago.
Here is the link to the Second Life Viewer that came out 3 days ago (give that viewer a shot with your setup):