Hello, Mike.
I have a similar PC build to yours - CS5 works extremely fast on my machine. You could try and increase the size of Lightroom's camera raw cache from its default 1GB to 20GB and see if you get any more speed in the develop module. Oh, and keep your 1.1 previews as well, as I've found it speeds things up.
I'm thinking of moving the location of my camera raw cache from the default C drive (which is a solid state 64GB, and is my operating system, programs only drive) to the E drive which is photos and other data etc.
The trouble is, I don't know how to go about this location change and what it might cause, stability wise?