I have an i7-2500k, 8GB RAM, a pair of Asus VH242H displays, an i1 Display Pro calibrator, and a Wacom tablet. It is fine for looking at photos but editing a 25MB .dng with lots of brushstrokes in LR4.1 can run it into the ground.
A faster processor and 12 or 16GB of RAM would be a better idea, as would one good monitor; The panels I have are remarkably inconsistent for brightness and color temperature.
The calibrator and the tablet are swell once you figure them out, applying adjustment brushes is much easier with the tablet.
The only other thing I'd change on this setup would be a faster drive array. I have for storing photos a RAID5 array of older mechanical drives on a proper 3Ware card but a bundle of SSDs would sure give it a kick in the pants. Win7 runs on its own Velociraptor, which is enough.
On the LR 4.1 issue.
I have a i7-950 Quadcore clocked to 4.02Ghz each core.
Samsung 830 series 6Gbs SSD (256GB)
GTX 460 series Graphics...
System is not a slouch by any means... Yet LR 4.1 exhibits sloth like behaviour.
I read somewhere that if you goto lightroom.exe in Task manager and right click on it and set affinity to the "even" number core only (the odd numbers are Hyper threads not physical cores) then LR 4.1 runs a bit better.
I would tend to agree... but it is still slower than LR 3.6. Adobe needs to fix this... Shame on them.
PS: At no time does my RAM usage exceed 8-10 GB while LR is slothing away...