btw, PS2 isn't forgotten at all. Don't forget that there are far more PS2 games around and far more users of PS2 then Xbox. Yes, Halo 2 will have some people changing to xbox, but still. Also, then new GTA (for PS2) is due to be released and in terms of users, this will be very close (if not more) then Halo 2 on xbox. So don't write of PS2, just because Halo 2 is the current hype!
Also, both consoles have reached there point where the graphics can't be improved anymore some time ago. And tbh, the Halo 2 graphics aren't that of a big "wow" in console gaming industry. In fact, graphic-engine-wise, there is nothing that has been improved significantly since Halo 1.
You want to game online for as little as possible money? you want the best graphics? You want the best compatibility/upgrade possibilities? Stick to your PC then, and invest your money in some more memory or better graphics card...
PS: also, when Halo 2 for the PC hit the stores, you'll see all kind of (free) mods also. Something which can't be done on an Xbox. Also graphics-wise, there are more possebilities with PC graphics then their are on an Xbox, so I wouldn't be surprised if the PC-version will look even better then the xbox version.