for your budget i would say Q6600 would be perfect
among other things would be 4-8 gb of RAMs, and 4x harddrives for RAID10 setup (this will improve overall performance as well as making backup for everything you will be creating which is best for work stations setups)
i haven't used vista so i cant say on my part about the compatibilities on it. but 64bit sounds good to me since CS3 is multi-threaded software so i assume it would run faster possibly??
ASUS P5Q Deluxe (P45 mobo) $210
4gb ram (2x2gb) $80
4xhdd (Seagate 250gb) $240 (all 4)
ATI hd4850 (800 processing units each) $170
750W Corsair power supply $110
Antec Nine Hundred case $120
DVD+/- (your choice) $30
AS5 thermal paste $6
if you have $ to spare for upgrades get one more HD4850 video card and then one more 4gb(2x2gb) for 8gb ram if you run in 64bit OS
I run cs3 creative suite on vistax64, and there are some funky compatability issues. Nothing too serious though, and I'm sure that'll be taken care of with the next photoshop release. The bridge never works perfectly, and the install/remove program is pretty much not vista compatable. That's ok though, untill you want to remove it from your system. I run a q6600 on a p35 board oc'd to 3.6ghz. It's pretty powerful for all of the adobe programs.
I second the system gaiden posted. If you're not going to overclock though, you might go for a faster processor, i.e. q9450. A p35, of p45 mobo will do fine, just make sure you get one with raid support.