To start you off, you can just add an extra hard disk in the computer that you already use. When you decide on the rest of the hardware you can plug the linux disk in the new motherboard and it will work provided you reset the video parameters.
With dual disks you would install a boot manager such as Grub that lets you choose between systems at startup time.
The amount of RAM/CPU speed etc depends on what you are actually doing. My linux firewall/router manages fine on 16MB on an old 90MHz Pentium and for that job the hardware is ample. My linux systems at work on the other hand all fast Xeon CPUs and 2GB, because they are all supporting three Win2003 environments plus large image processing applications each under VMWare.
For the usual range of web browsing/ email/ wordprocessing etc I think 256MB or 384MB should do well. You may need to add more if you are also going to run other services like a database or a web server.
I have not seen people running into problems with chipsets with recent distributions of linux -- not for a couple of years now.
One important thing before you install is to decide on the most efficient partitioning scheme. In unix-based systems the concept of C: D: and other drive letters is not used. For a single disk, I prefer to have four partitions --
/ -- About 10GB
/var -- About 10GB
/home -- Rest of disk less 1GB
swap -- 1GB
I'll fish around in a bit to find some more helpful documents. It's not as difficult as you might think, and it is very pleasing to be able to get control back over your computer rather.
Rather than guess, there is a hardware compatibility list somewhere. In general the very very very newest bleeding edge stuff might not be supported properly by a current release (although normally after a month or two, drivers etc become available). You might also consider that more people remark on having "adventures" getting it to run properly on laptop kit than on regular desktop hardware.
j
[small edit to clarify a couple of points]
[This message has been edited by jooles (edited 06-25-2005).]
