Save the money that you would pay for an MMO, upgrade your connection, and buy whatever free MMO you would like.
Haha, i dunno. I am currently on a trial run of WoW, and own and played GuildWars(lvl 18 war). GuildWars was awesome because I wasn't paying for it, and i felt like I got what I paid for...just the game.
WoW though...hMm....not sure yet. Like I said, i'm on trial right now. Will probably sign up for 3 months and check it out. Have to see if it's worth the money.
I have a friend on 56k, and he plays just about anything. LIke mentioned before, a lot of problems when a lot of people are in a certain area(Ironforge or whatever comes up...frequently when peopel complain about it), however, generally as long as your computer has the right hardware to run the game smoothly, your 56k should be able to keep up as well. Expect to have some bad experiences every once in awhile though, sometimes if the servers are slow, everything just goes to hell.
Really though, you could pay a few bucks a month and get something faster than dial-up, and get a free mmo. there are a few of them out now. Isn't there Dungeons and Dragons now?(anyone???)