I would think a Dual Core Dual would handle it fine. I would look at what is running in the background while working and try to run your progrmas with nothing going on in the back ground, like email and music. I'm a PC guy, but I do havethe 7600 and dual core duo and run LR without any problems
Unfortunately you've got 3 factors that will limit aperture 2 performance -
1. The Nvidia 7600GT GPU. This isn't upgradable unfortunately...
2. Only 4gb of ram, aperture tends to page out until about 6GB or so. You probably can't put more than 4gb in the white imac.
3. Access to more than one fast internal storage drive. The library is disk intensive and benefits from being on a separate volume than the raw masters.
Lightroom is pretty much always going to run faster on your hardware than aperture 2.
On the other side of the coin, on a machine Without these 3 issues aperture 2 is still quite a bit faster than lightroom 2 and 3 on my hardware - (mac pro 2.6 and 3.2) - it is about equal on my unibody 15" macbook pro.
Use Lightroom (the Lightroom 3 beta is terrific and free for the time being). and store your image library on an external Firewire connected hard disk drives instead of on your internal drive.
If I were you, I'd try to hang on for another 3 months or so to see what a possible new version requiring 10.6 might bring.
I also had a 24" imac and noticed that I started pushing it once I upgraded from my 20D to my 5d2. It is still possible to do a lot of work with quick preview mode and thus it was still a faster workflow for me than lightroom, but things could start chugging with the right combination of adjustments.
My iMac is limited to 3.3GB RAM even while being able to recognize 4B in the About this mac page.
I would also consider the Hackintosh way as right now the pocket is not for spending 2000 bucks on a new Mac.
I guess the only option for me would be buying a second hand 8 cores Mac Pro once they release the new ones. The new iMac while being a very capable machine, does not allow a second hard drive or USB 3.0, e-sata. Just Firewire 800 and that is not fast enough for my future needs.
I guess I will need to wait quite a while to get it and be patient.
A previous generation tower is not a bad route - I got lucky in june and was able to get a brand new unopened, not refurbished, warranty from an authorized dealer... 8 core 3.2 2008 model for $2699 with the nvidia 8800GT.
If you are pricing for a new machine I would definitely consider what openCL brings to the table. The ability to swap out the graphics card in a couple years could essentially give you an entirely new aperture Rig...
This was the primary reason I decided to spend the extra money on a tower. I don't have any knowledge about aperture 3, but the writing is on the wall for what to expect, and I do have knowledge about openCL... So I would call this decision an "educated guess".
Unfortunately as mudsill noted - Expansion card slot technology often changes over a 3-5 year span - but hopefully since we are on the intel platform now we will be with PCI Express 2.0 for a while.
The secondary reason was 4 internal drive bays (6 if you remove the internal DVD drives). And 8 if you use a 3rd party bracket with SSDs which would make the system FLY.
I am at the point in my part time/semi-pro photography where I have less time and more work to do. Upgrading the system was a no brainer and paid for itself within a month since time is money and this was easily 3-4x faster than my imac in "real world" hours I spend on a project.
$2699 could be considered a lot of money, but it's only a fraction of my 5d2s, my 50L, my 135L, my 16-35L...
matthewbmedia wrote:
Just a note, you can't "upgrade" your library with a trial version, but you can "import" your library through the file menu to try it out.
The implementation of "layer masking" is pretty amazing. especially when you start painting on/off a highlights&shadows brick.
Small warning - automatic preview generation, and automatic places and faces processing still slows you down at the start.
However making adjustments is probably 2-3x faster noticeably on my mac pro.
I have to agree with that.
I before had big troubles with the brushes as the computer slowed down quite a bit but now I see it going very fluent and no stops when used intensively.
Polarizing brush and the contrast brush are just great. I had tried with a batch of 20 pictures and it takes around half the time and I used brushes while before I could not lift the brushes. This is a huge difference, I'll be able to hold on to this computer for a while more.
but my eyes... 
