*** Lots more to be added - would love a Mac person to add some ***
In this article, I am assuming that you need a good computer for doing Photoshop or similar work.
First, there are two main types of computers people buy - Macs and PCs. Apple makes the Mac and many companies make PCs. There are pros and cons of each.
Pros and cons of the Mac
Macs are probably more popular among people who are into Photoshop as a profession. Macs run on well designed and streamlined operating system that gives them a significant advantage over PCs that use Windows.
Macs look good which is nice when setting them in your home office.
Pros and cons of the PC
PCs are the most common computer today. They are cheaper than Macs are are generally more expandable. There are more accessories for PCS and more software for PCs.
After you choose to go with a Mac or PC, you need to decide what options to get.
Here are some considerations:
Memory
I think that memory is the most important consideration. For a PC, the most you can get is 4 Gb with a 32-bit OS. If you get Vista x64 you can get more than 4 Gb. Further, on a 32-bit OS a single task can only use 2 Gb at a time whereas in a 64-bit OS, more than 2Gb can be used by a single task.
On a PC some of the memory between 3Gb and 4Gb cannot be used in a 32-bit OS. So, if you get a PC with a 32-bit OS, it might make sense to get 3Gb.
Hard Drive
There are many considerations here. First, in terms of speed, the most common measurement is RPMs. Slow speeds are 5400. Better speeds are 7200. Very fast speeds are 10000 and 15000.
Try to get the 7200 RPM Hard drives as opposed to the 5400 RPM ones.
Next, is how much to get? This depends on how much you intend to store - obviously. Photographers can generate many Terabytes of raw files in a year. They need a system to keep what is important but to not keep garbage that is costly.
Also, keeping all of your work on 1 hard drive might not be a good plan. It is probably better to archive off old work to other places and only keep your current work on your main computer hard drive.
Archiving is probably a separate topic - lets just say that a plan is needed or eventually it will come to tears.
Adobe recomends that you have two hard drives on your PC. One for the software and another for your scratch file. So, if you bought a PC with two 500 Gb hard drives, that might be better than one 1 Tb hard drive.
Processor
There are 2 brands of PC Processors - Intel and AMD. I think both are good. The main way a processor is mesured is by clock speed - like 3 Ghz. Another important dimention is how many cores - 1, 2, 3 or 4. A quad core processor is like having 4 single core CPUs. It means that 4 tasks can be done at the exact same time. There are other considerations and limitations but this is the simplest way of looking at it.
******Thoughts on quad core processors and Photoshop?
64 bit OS vs 32 bit OS
A very important development in the PC world is Windows Vista x64. As noted in the memory section above, there are tremendous advantages to the 64 bit OS. However, there are two things to remember about x64.
First, many programs will have bugs or not work. Computers with x64 are still rare and if you want a problem free life, x64 may not be for you.
Second, many pieces of software are still 32-bit and will work in 32-bit mode on your x64 machine. They will still work but won't be able to take advantage of the OS. One important piece of software is Photoshop CS3. It will only run in 32-bit mode. Adobe Lightroom 2 will work in 64-bit mode. Photoshop CS4 due in 2009 will work in 64-bit mode only for PCs (not Macs).
Video Card
The video card will not speed up Photoshop much but it has a bearing on you screen. You need a video card powerful enough for the resolution of the screen that you choose. If you choose a 30" screen with a 2560x1600 resolution, you will need a reasonably powerful video card. If you have a 22" screen with a 1680x1080 resolution, you will be fine with a lower priced video card.
Monitor
Laptops versus Desktops
Other Software