For most of the online FPS games I play, it's usually HammerFall and Blind Guardian, maybe Korpiklaani. And hell yeah, this increases my overall performance like nothing else. For other games, usually just OSTs.

For most of the single player games I tend to stick to the original soundtrack if it's good, otherwise I just mute the music and put something that suits the atmosphere the game is trying to (re)create.
During multiplayer I am usually to busy shouting in my mic and listening to the ranting of my team (a usual player of team/co-op mp) to care about music. I also prefer hearing that damn spy coming up behind me than the voice of Nerina Pallot for example (first singer that came to mind, I actually love her voice/lyrics/music).

If the game is either plotless or has long sequences with no plot (such as grinding/farming in some RPGs or dungeon crawlers) I put on books on tape or podcasts. However, this is usually (not always, but usually) reserved for games I'm playing for the second time. The first time through I prefer to immerse myself more fully in the game, and the in-game music is an important part of that, IMO.

Quick poll: How many headbangers we actually have hear? ;)
Personally I like to listen ingame music and if the music isn't good enough, I just turn it off.
i.e. Dune1 has very good music and fits perfectly for the game and same goes to Deus Ex but some games like Duke Nukem 3D ... god damn. Who they wanted to punish with that!?
When I was younger I did overrided sound track sometimes just because I had CD-ROM and my friends didn't ;) Normally I used Manowar, W.A.S.P, Megadeth or something similar.
Ah, yes... I can still remember my friends face. He always beated me at Warcraft 2 but then I spent few days and studied that game and next time I put Manowar on and started to grow Paladins... When it was time to kill, I unplugged my headphones and let the speakers play it loud... "Sound of charge, Into Glory ride..." :D I won that battle. 6x9 paladins from 3 directions...

I'm usually satisfied with in-game music. Especially in single player when well chosen music can boost the atmosphere in some moments.
But when music becomes repetitive - in race games or on-line multiplayer games, I usually turn Winamp on and tune something harder like ChroniX Radio or Heavy Metal Radio
But now I'm listenig The Witcher: EE OST - "Inspired by" disc while browsing.

When i play games with good music, i leave it on.
Otherwise i try to find music suited to the atmosphere of the game. Quite often, good film or series music fits well in games with the same theme. I especially like Ennio Morricone's tracks (The Good, the Bad and the Ugly,...).

Really depends on what I'm playing. If the game tends to be a bit more of a grind, I'll usually listen to metal or hip hop while playing. This was typically when I was playing WoW or something like GTA.

I like to listen to Slayradio when i'm playing games. But that's if the music in games doesn't fit the game itself

Most of the time the in-game music is suitable, but if themes get repetitive, I just reach into my computer playlist and try to grab something that fits the mood of the game, somehow Techno always seems to work with most games...

Really depends on how I feel about the music from what ever game it is I am playing. If I like it I will just leave it as is(Fallout, Deus Ex, KOTOR, etc.) However, if I don't like it or get bored of it quickly(lets say WoW) I generally switch to the music to either what ever I am feeling like at the moment or just listen to a podcast.

for me it always depends on the game. for example, over the dozen of so times i've played the fallout games the sound track is always the same.
Tool Aenima.
other games "feel" like other albums to me, and i'll adjust accordingly.

Whatever the game soundtrack is.

Usually I leave the game soundtrack on, but if it's a game with a limited soundtrack and I've been playing the game a lot, I'll find an appropriate playlist.
For instance, when playing Jagged Alliance 2, I usually load up my 15+ hour playlist of Frank Klepacki soundtracks and randomise it. Fits perfectly.

For some weird reason most of my favorite games already have kickass music. Fallout for instance. But sometimes I still put on something else. Mostly music I want to listen to at the moment, regardless if it fits or not. Mostly Tool, Mars Volta, etc.

When i was younger, I could'nt find my earthbound cartridge anywhere, so i found the wonderfulworld of emulation. Back then my computer was a hunk of junk, so i had about 9 songs on my playlist looping. I can't remember any of them now, but if i ever hear them on the radio it gives me an instant nostalgia boost, which is quite nice. (off-topic nostalgia speak) the other day i saw a shade of green that reminded me of Megalomania.