virtualbox.org

Right now, I have a system with a Socket A motherboard (Athlon XP 2600+) that has Debian installed with VirtualBox running Windows 2000. I'm using Nikon Capture for photo editing.

I'm able to test using a system with a Socket 939 motherboard and the processor is AM2 with a 4200+ CPU. I was wondering if anyone could inform me of the relevant speed and performance differences. I would like to increase the speed significantly and was wondering if the later computer would do so or whether I'd need to invest in a more modern computer.

The speed of the guest OS is dependent on 3 major factors:

CPU speed which is around 90-95% the speed you get on your host system

RAM "speed" is in almost all cases equal to its size. You need what your host OS + Applications needs to run fine and on top of that what the guest OS + Applications need.

I/O speed (HDD transfer rate etc) is limited by your hardware and should also be around 90-95% of the speed you get on the host.

3D Graphics is always way to slow on what ever hardware the host is running

Now you should ask yourself what you would like to improve compared to your current system.
Photo editing only needs CPU power and RAM, but it really depends on how fancy you get (and how capable the program is).
Any not to old CPU (<2 years old), 3 GB RAM (1-2GB RAM host, 1GB RAM guest) should be used today.

If and only if you already own the described "new" system I would upgrade, as it should be much faster in any respect (if it also has more RAM than the "old" one). If you have to spend Money on it think twice before you do, even if it is cheap. If you need to upgrade you have to consider that the 939 System uses DDR1 RAM which is much more costly as DDR2 RAM, it may supports only older Graphics Card with AGP and CPU upgrades are not really economical anymore.
You may can double the speed of your current system but this is also true for a current not-low-end-system compared to your "newer" one.