I am on a mac and always have been... it is an older G5 quad with 12 gigs of ram and plenty of harddrive space. It is running everyrthing up to date it can. Photoshop CS 5 and the new LR will not work on it.... I will getting a new Macpro but I want to get the right one.
Well it all depends, We don't really know how well exactly PS CS5 and LR3 will use multiple cores. There certainly are programs like C1 (if it would not crash every few minutes on windows) and PTGui which actually can use all of my 12/24 cores/threads.
However there is the aspect, that lower core number means more speed. I know that when a program is designed to use a lot of cores it will be faster than a fewer cores. Even if every core runs at a lower clock speed. However I'm not so sure when it comes to PS and LR. I can't say whether 2 cores at 3,6Ghz, 4 cores at 3,1 Ghz, 6 cores at 2,8ghz or 12 cores at 2,5Ghz would be faster.
However there are a lot of more things than CPU. One thing is RAM. If you only work with 1DsMk3 files and don't stitch or make use of lot's of layer, something around 6-8 GB should be more than enough.
One good thing would be to invest into a SSD for your main OS drive.