Ive been lurking here for a while waiting for a solution and in upgrading to win vista ultimate sp1 I made a discovery, with the "generic" Microsoft display driver for my 8800 GTS, CS4 runs like a dream, with absolutely no lag during the circle test, also tried opening a vector image with several hundred layers I had been working on, and it was open in a second, no lag in zooming or panning. This is all without GPU enabled, it was greyed out in preferences. After I installed the Nvidia beta drivers (180.84) everything got laggy and slow as it has been from day one when I purchased Master Collection.
Also I have done a few clean installations to try to see if that was the problem, installing Photoshop before anything and testing after installing things. The other place where I noticed a small performance issue is after I installed the Wacom drivers for my Intuos and Cintiq there was slightly more lag with the brush tool. This time around I upgraded from Vista 64 home premium to Ultimate to take advantage of my second processor (2x dual core xeons) for some reason Windows let me use all of the cores with home Premium until started tinkering to fix Photoshop, so I had to upgrade to get them back, unfortunately it having the extra 2 cores makes zero improvement. I'm holding off on buying more ram too, using 1GB sticks I can go up to 16GB, but right now it doesn't seem like it will make CS4 move faster, so I am going to save my money, like I should have done with CS4 in the first place
specks:
Vista Ultimate SP1 64 bit
Dell Precision 690
2x Xeon 5060 3.2GHz
4 GB ram
EVGA 8800 GTS 640 MB
21" acer (1680*1050)
Cintiq 21UX (1600 * 1200)
Intuos 9*12
300 GB Velocoraptor 10,000 rpm for system drive
36GB and 74 GB Raptor dedicated scratch disks
AHCI is not enabled yet, issues with detecting them during install. Also no RAIDs or anything like that, maybe later with the 36GB Raptor as I have a spare.