What size system for CS4/Aperture?

If you can wait two or three months, Nehalem is coming out. The benefits are... decently huge.

 

What TS said. And if you don't get an i7 system, it will be a great time to pick up a previous generation Mac Pro system in the refurb store at a great price. Get at least 8GB of ram whatever you do. Good luck.

 

Future versions may make better use of more cores, but there is no guarantee and depending how you work you may not need more than 4 for the life of your machine anyway. You might want to consider a refurb too as such a system can be bought for $2,000. Add 8 GB of memory and a faster hard drive and you have a solid system for the next several years.

 

For reference, I'm running Aperture 2.0 and CS4 on a 8x2.8 Mac Pro and it's plenty fast, so you could easily get away with a single CPU 2.8. So, when the new Mac Pros hit, you can get the bottom end one and be fine.