for playstation 2 online
 

I can connect any system to my computer, so will I be able to use the modem on my computer for ps2 online, or will I have to buy a seperate one? someone give me a full explanation of this bitch :(

You'll need a router or at least a crossover cable.

already got a router connected to two computers, so I just need to connect that to the ps2?

also, how much does it cost

Yeah, just connect your PS2 to the router. It's like 40 bucks for the adaptor I think.

Yea, network adapter is 40. There isn't a subscription fee or anything like XBOX Live, except for maybe Final Fantasy XI (not sure).

instead of putting it to your second computer, put it to your ps2

Did your ps2 come with the modem on it? Then you just connect either the phone line or ethernet line to it. If not then you need to buy the network adapter. Like so.
http://scea.com/peripherals.aspx?id=SCPH-10281