Depends how tight of a ship you run.
If you are anal, and know how to correctly install a copy of win98 and not really any other apps, but games, then you will have the most speed here. The only way I would recommend this is if, you do a superb install of win 98, tweak it a little, and don't install anything else but the games you intend to play. Every app you install in win98 slows it down and increase its likelyhood of crashing. This will be your fastest option. I have done this before and have loved it. If you do anything else besides games on your computer then dual boot it. Make sure you keep your win98 install clean and no other apps. It must remain pure. If you keep it pure, then it will be as stable as 2k or xp, but with really not other functionality besides alienating your girlfriend .
2k. Well 2k, I can't really speak for as I never played with it. Most of my friends have it, and have loved it. Now they all use XP and love that.
XP. Think of XP as the best of both worlds. You will get almost as much speed as a pure win98 install, but you CAN install other applications. I love winxp, user friendly, fast, stable, etc. I can have all my games installed, icq, aim, office, visual c++, jbuilder, and whatever tickles my fancy, even the crappy shareware/spyware I have to install on occasion doesnt wreck it. If you want to wreck a win98 install, then install as many shareware/freeware file sharing apps as possible. This will strangle it, not so in XP.
I would go with XP personally or a dual of 98 and xp, depending on how tight u can run your ship and if that extra 5 - 10% of speed is worth it.
If you want the fastest possible then create a hybrid 95, 98 install. It removes the IE bloat of 98, but keeps the infintely better memory management of 98 and everything else better about 98.
http://www.98lite.net
I use this when speed is the only option. Usually if I want a fresh install for a lan party or something. It simply smokes xp, 2k, and is better than just 98.
Pax.