All you need is a free AOL account or something similar. iChat has that capability built in. Haven't used it in maybe a year, so I can't be much more help. The texting method can probably get by with a dial-up connection, of course. But the image/video method will obviously require a high speed connection. It can also be used to transfer files between viewers, of course. One nice use is a three-way meeting, but that does, I think, go easier with all three using a Mac.
RHP and
Highmac are probably the experts here on iChat.
BTW, Mobile Me and "dotMac" are one and the same, now. My granddaughters picked up a Jabber account for use with iChat, probably many other sites will work, also. iChat might even have a list of compatible domains in its Help pages...OTOH, most "Help" pages are very "helpful"...