Switching to Linux checklist
A. Download the ISOs. (RedHat 9.0 Rocks!... though some consider Mandrake to be easier to install and use.)
B. Make sure you have a good deal of spare time. Linux is not Windows, while you can configure it from GUI, most tasks are much easier to accomplish using the command line interface / terminals (Command.com / cmd.exe like interface). While the learning curve can be daunting at times, but once you get going, the reward is well worth the effort!
C. Decide, in advance, that you don't give up. Period.
D. Google! Assume that your questions have been asked (and answered) 10,000 times before. Make sure you've got a good Linux forum in advance. Don't be afraid to sound stupid. We all made some amazingly stupid mistakes in the past. (Anyone said rm -rf /)
E. Make sure you've got a backup machine (running Windows) that has an Internet connection. It's much easier to ask questions in google or in the forum when you don't need to dual boot for it.
F. Make a note of everything you do. You may need it in the future.
G. Good luck!

Supported Hardware

CPU: Pentium, Pentium II, Pentium III, Pentium IV, Xeons, K6, Duron, Athlon Thunderbird, Athlon XP, Athlon MP.
Chipset: AMD, Intel, VIA, nVidia nForce. (The latter may require drivers downloaded from nVidia).
AGP: nVidia Geforce based recommended.
2D support: ATI Radeon 7xxx, 8xxx. nVidia. Most legacy cards.
3D support: nVidia Geforce based. (Using drivers downloaded from nVidia.) ATI 3D support is still lacking (at best). Most games (Q3, RtWC, UT, UT2K3) will only work on nVidia cards.
SCSI: Most (if not all) normal / RAID cards are supported.
IDE: Most IDE Raid cards are actually "software" raid. There is limited support for most raid 0 cards. Raid 5 cards will require downloaded drivers.
Modems: Hardware modems only. Most Winmodems are a no go. (Unless you can download drivers.)
Network cards: 99% supported out of the box. (At least in the chipset level... which should be enough)
USB/2 cards: All the major chipsets / protocols are supported. Should be enough.
USB devices: Depends on the device. Some supported (zip, scanners, printers) some are not.
Printers: Both LPR and CUPS have a very long list of supported printers. RedHat 9.0 uses the CUPs subsystem, which should support 90% of all printers.
ADSL/Cable modems: Most will work. Will require some work to get them working. My suggestion? Get an external modem and connect to it using a ethernet card. (Plus, it will work faster and eat less resources.)

Courtesy of Gilboa Davara (AMDZone and 2cpu)