Save your money and get a more recent Mac Pro, the extra money for the CPU and RAM difference is well worth it.
I was thinking the 2.8GHz would be a CPU swap in an '06 system, not an '08 model (which makes much more sense for firmware reasons).
Are you suggesting an SSD? I intend to run a 64 or 120 as my boot disk.
I am just looking to have something DRASTICALLY faster than my 2.33ghz core 2 duo mbp.
I plan to upgrade to 8-16gb ram + ssd boot drive
Let me clarify on the MP. I was assuming you meant to upgrade the CPU's in an '06. If you meant get an '08, it's the better way to go, but more for the firmware (EFI64, as the '06 - '07 is EFI32).
The reason is, EFI32 is already limiting the ability to upgrade graphics cards (they can't use nVidia's latest offerings, and certainly not those that might come out). So far, ATI's are working, but there's no guarantee that trend will continue.
It also affects the ability to run future versions of OS X as well. Currently, OS X has been running 32 bit Kernels, while SL has both 32 and 64 bit Kernels (they're trying to go back to full 64bit systems). As Apple doesn't stick with interim products, SL is more likely to be the last version that uses K32. Assuming they can stick to such a time table, 10.7.0 won't work on the early systems (EFI32 based units).
That means you'd need an '08 or '09 to get an EFI64 system, and the '09's are expensive. They also have some nasty limitations (may apply to you, especially memory - limited DIMM slots, ICH10R throttled to ~660MB/s - notable if you want multiple SSD's, and possibly internal RAID).
Given your usage, the increased clocks may not help that much (it still helps though, but sometimes the funds make better sense going elsewhere, such as drives). 3D rendering is what really benefits you in terms of running multiple cores (multi-threaded apps). PS as it happens, can only run 2 cores ATM. The software is in need of updating. Safari is single threaded.
What I meant about drives, is I was trying to determine the throughput's you actually need for your data files. SSD is nice for booting, but not the most practical for working data, as the capacity is too small. It's not impossible, just expensive to move to full SSD arrays now.
This is where mechanical drives still have a major advantage. RAID in the '06 - '08's are far easier than the '09's, and less expensive (especially if you want to use the HDD bays, as there is an adapter required for a 3rd party RAID card if you need to run one).
Further details would help.
