Hardware Issues

When installing a server style operating system (such as UNIX, LINUX, NT etc), hardware can be a major stumbling block. It is usually best to decide on your operating system and then check to see what hardware it supports, and THEN buy/build the system you want. Thankfully, as long as you don't buy "cutting edge" hardware, your odds of being able to get it to work are steadily increasing as vendors and programmers improve their hardware support.

Some things to be aware of with Linux and hardware:

Modems- "WinModems" have removed most of the hardware from the card and let the CPU do all the work. Which is extremely silly on most Microsoft Operating Systems as they tend to overload the poor CPU as it is, and they just are a waste of time in UNIX/Linux, even if you can somehow get them to work. Buy either an external modem (make sure your serial ports can handle the speed) or an internal from Zoom, or make sure it has the "Rockwell Chipset" on board. You'd be amazed, I sometimes get better actual speeds with my 28k 'real' modem than someones 56k "No Win Modem" and they try to do some other task.
USB or Universal Serial Bus- Considering that the standard for the MS Windows group continues to mutate, it's no surprise Linux has problems with this, though standard support for USB will show up in the 2.4 kernel versions. BEWARE: USB also has the CPU do a lot of work, and still isn't quite as fast as SCSI cards.
SCSI- Definitely the way to go for many devices (scanners, Hard Drives, CD ROMs, RAID, Jazz drives, CD RW and more). The speed is truly a wonderful thing. But be careful, support for different SCSI adapter cards varies WIDELY, and Redhat Linux no longer supports SCSI during installation very well, if at all.
Printers-Another area that can be disappointing if you are not careful. Look at the list of supported printers before you buy if you can, otherwise you'll have to figure out which supported printer is close enough to your own to work. (Either that or find a third party company that provides the proper filters for your printer)

What you need to know about your hardware
To run a UNIX/Linux system, there are certain bits of information that you need to know, especially if you want to optimize your system.
For each card you have in your machine you need to know:

IRQ (interrupt request #, some plug and play cards make these change randomly with each bootup)
IO (input output memory address)
DMA (direct memory access channel, often important for sound cards)

Monitors and Video Cards are another possible place to have problems. It can help greatly to know the following:

Video Card

Exact Model of the Card
Chipset
Clock Settings
Amount of Memory
Video Resolutions that it Supports**
(be careful not to overdrive your monitor, you can do actual physical damage to the monitor)

Monitor

Resolutions Supported
Vertical Refresh Rate in kHz (very critical) at different resolutions
Horizontal Refresh Rate
Either exact model, or which models it is synonymous with from the same manufacturer.