2-3 Xboxs, One PS3.
Anyway, Killzone 1 HAD online play. No doubt the sequel will aswell. Also, PS3 supports seven Bluetooth controllers out of the box (pray to god that Bluetooth doesnt have Latency problems, but if it does, then use 3 wireless controllers and plug the remaining four into the USB slots on the front) so you don't even need a multitap. Halo is just overrated anyway <_<. Killzone 1 wasn't all that great, but the online was fun as hell.
Also, Kutaragi and Honda confirmed a $375 dollar price for Japan. Usually, about 60 dollars gets shaved off the Japan price each time a new console came out, so I expect something like $300 or $350.
AND, Phil Harrison and some other guy from Sony both confirmed on G4 that Killzone was in-engine and in-game. They didn't even try to avoid it, they said it at least 2 or 3 times that it was what you will see when you pop it in at home and play. Yay!