Well if you plan to network via ethernet you'll need a cat5 crossover cable. Or, if you have an ethernet hub or switch, that will work too. Otherwise you'd need some sort of inbetweener device like a Zip drive, Jaz drive, external hard drive, or you'll have to burn a mess of CD's (if your old iMac is one of the CD-RW/Combo models). Otherwise... I dunno what to tell ya! Good luck to ya... and congrats on the new iMac!
AS Hemingray stated, either the crossover cable, or a networking hub/switch. Do you have any friends with either item?? If not, then just go to your local computer store, or uber-electronics shop (we have one called you-do-it electronics that has tons of stuff, as well as crossover cables, and other networking hardware). You will also need to share one of the drives in order to move the data. I suggest sharing the one you are pulling the data from. That way, you just need to drop it where you want onto the new computer.
Another option, which will cost you more then the networking cable, would be to make the hard drive from your old iMac external. Put it into a firewire enclosure and then just mount it as another drive.
I think the x-over or hub/switch method will be both easier and cheaper for you though. You should be able to get a 6-10' crossover cable for $10-$20. It's been some time since I had to get one, so I don't have current prices. Once you have it though, you should keep it, just in case you need it later.