For iLinking up 2 PS2s, you need a 4-4 firewire cable. You should be able to find one for about 10 bucks at most (though depending on what length you want).
For iLinking up 3 or more PS2s, you need a firewire HUB. Cheapest are those with 6 pin HUBs (also more solid than the small 4 pin connections, get them for as little as 40 bucks if you search well enough). For each PS2 you want to attach, you'll then also need a 6-4 firewire cable. This solution can of course also be used for 2 PS2s.
You are limited to driving the Arcade cars in iLink mode. But for multiplayer games it still rocks. In 10 days I go to one of the biggest iLink event ever, the RPC (IV) in Bochum, Germany, with possibly over 50 people attending. A month later, there is a big event in Lyons, France. I'm thinking about going there, but it's a lot further away, so I'm still thinking about it.
I've attended about 10 iLink parties now I think, many of which I've organised myself. I have two iLink HUBs at home, as I also discovered that you can hook up 8 PS2s for one game - two of the 8 will then be like track/replay cams with which you can watch (and record if you attach it to VCR, DVD-R or PC) the race as if you were watching a replay. It's absolutely awesome especially if you can find some other drivers that are close to your level.
