Here's some feedback on the i7 system with respect to video cards and the brush performance.
I'm running it on Vista 64 Ultimate with no other apps.
First of all, Brushes. On my home machine, an AMD single core with XP pro 32 bit, I have two problems. One is the truncated circle, the other is the bad gray value when cloning in an 18% gray range. It would switch automatically from a black circle on white, a white circle on black, and an unfortunate, fixed gray on gray.
Different matter on the i7 machine. No truncation, but there is only a black circle no matter what the background color is, until I hit caps lock, then hit it again. The circle now changed to white, no matter where it is placed, but tries to shift to another color when the cursor is moved. Exit PS and open it up again and the brush reverts to the original condition. Not very promising!
As to the video cards, I found a rather startling result, given the wide spread nature of performance among video cards/drivers. I have none.
I have my own card on hand, an nVidia 7300. At the lab, we have 7200 and 7100 versions laying around. I have the latest driver installed for the 7 series. What was starling is that it didn't matter which card is installed, the system showed none of the problems discussed throughout this forum. That is, I could substitute any of them and if there are subtle differences, the drivers reload for that particular card (or corrected for the card) and I see no change in anything, including the a fore mentioned brush problem. IOW, the system is completely stable with respect to swapping 7 series cards, and if the brush problem is video related at all, it follows the card as well.
This is true also when trying out different platforms/cpu combinations. All I need to do is swap the HD among them, assure that BIOS is set at default (and in one case, set to a significant overclock) and run.
So, there you have it. So far. If I absolutely had to have CS4, I would invest in the Intel board and the 2.66GHz processor. No need to spend for the faster ones, overclock is simple and pretty stable (up to a point!) But really, I didn't even know that one of the boards was over clocked until I did a BIOS check before closing out the test.