I envision using that new machine for the next four years or more ...
I'm a self employed graphic designer. I usually run at the same time:
my design apps like PS, Quark, Acrobat, (sometime Flash and Dreamweaver);
my internet apps like Mail, Firefox and Safari with a lot of tabs open;
then my entertainment apps like EyeTV and iTunes (with a huge music library).
The only speed/performance problems I run into lately with my old G5 is iTunes (can't handle huge libraries - hello beachball) and Firefox/Safari with lots of tabs open.
So I'm wondering which one of these new models will give me the best value. I don't think I will do any video encoding or 3D rendering in the foreseeable future.
Do the multiple cores help me with running all the above mentioned apps at the same time, or is a faster processor more helpful?
I'm skeptical about the RAM restriction on the quad-model, otherwise I would go for the bottom end 2.66 quad-core with 8GB RAM....
So any suggestion for all the non-audio/video users out there?
Thanks
Several things to keep in mind:
1. How much multitasking are you doing on the computer. Just having 4-10 apps open doesn't necessarily that you are leveraging an active multiprogram workload.
For example: Flipping between Safari, Mail , Quark probably means that only one of those programs has lots to do. Which ever is frontmost and getting user inputs.
versus
A batch job of Photoshop filters going to apply to 100 photos going on in a non-user-interactive session while same time running itunes for listening music, EyeTV digitizing TV shows, the Software RAID 0/1 striping your disk drives , and running Quark in the foreground with no visible slowdowns.
in short one mainly has single user driven workload and is more similar to if multiple users were logging in actively doing different things.
Humans primarily just do one thing at a time. That's why there is just one menu bar.

[ We can fake multitasking but that is usually quite limited and no where as good as what microprocessors can do. ]
2. Virtual Memory versus instant gratification.
Related to above. If have applications that are just open and not actively trying to do much work, then they can be paged out by Virtual memory. The Quad s have a limit of 8 GB or real memory. You can page out much more than that. The "cost" is that the application will "wake up" a little slower if don't use it for a long time. (e.g., Firefox needs 3 seconds to page in that page I haven't looked at in 4 hours. ... not really a big deal to me.) If that means money in your pocket for periodic waits for apps use sparingly.... that may be a trade off.
In short, you need enough real memory for the applications that have to actively run concurrently and you need rapid response from.
[ some folks think they need rapid response from everthing and that paging is never an option.... that firefox page from 3 days ago must appear on the screen in subsecond time or the world will end. ]
3. The Quads may eventually take 4GB DIMMs. Right now 4GB DIMMs of the type that the Mac Pros specifically need, cost a significant fraction of whole rest of the Mac Pro (i.e., would push it into the 8 core price range. )
When those prices come down out the the stratosphere ... Apple can change the specs. ;-) Right now it doesn't even make sense to even offer the config now because it would be cheaper to buy the 8 core and get what you want (12GB , etc. ) with 2GB DIMMs.
If you have primarily single user driven workload and a sub 6 GB average working set of data activitly working on with "instant" response times, then the Quads may be good enough. 2 years from now may be able to push past the 8GB limit without a problem.
[ If you primarily use apps just one at a time then the higher single core clock speed will help unless the 2.93 box is affordable. ]
If you have 8 cores of work. (you could do even more now, but don't because don't want to swamp your current machine. Example, hardware RAID now vs. software RAID on new box ). Or 8 cores will earn you more money quicker. ...