You can get started by playing around with one of the various LiveCDs, which let you boot a GNU/Linux OS off a CD and use it without altering the contents of your hard disk. I personally favour Ubuntu http://www.ubuntu.com but see the sticky thread on choosing a distro.
As to what you can do with it; pretty much whatever you like. Games are, on the whole, a bit pants or subject to the vagaries of WINE (a Windows runtime that lets you run Windows programs on Linux).
Generally, there is a good replacement for most Windows apps. Some examples are:-
Well, as bluetracer said, you can start using live cds, my personal experience was a hard disk install of suse
Youll eventually format and reinstall windows anyways, so, in one of those reinstalls, you can make a small install of any linux flavor you want
You can also check the choosing distro thread
I've used SUSE 10.1 (amd64) suse 10.2 (x86), FC6 (x86) dapper (x86) and currently feisty (amd64)
Suse had a LOT of problems...maybe because it was my first encounter with linux...but I had a lot of troubles with suse, I couldnt play video files, the ati drivers gave me problems, I couldnt make beryl to work...
FC6 was terribly SLOOOOOOOOOOOOOOOOOOOOOOOOOOOOW... so I wiped it out quickly, both ubuntu versions I have tried work ok, right now I only have one major problem with feisty, is that whenever I start ubuntu, all of my other partitions are unmounted...
I have also tried DSL but just as a "wohoo, the 50Mb OS " kind of test...
If you want to play games on linux...well, WINE but nexuiz and UT2K4 are great games to begin (if you like RTS, homeworld can run on linux too, but I dont know if the macross DYRL mod works under linux )