You need the same kind of charger as you returned, but instead of the USB one, you need the firewire one. Apple sells these.
The problem with not having a firewire port on your computer is that if the iPod is connected via USB, it won't charge whist it's transferring songs. This is a very large drain on the iPod's battery and you'll find that the battery runs out in the middle of the transfer (especially if it's a lot of songs you're transferring).
There used to be a cable that had both firewire and USB plugs on it so you could plug the firewire end into the wall charger and the usb end into your computer and so get power to the iPod whilst transferring data.
I haven't seen one of these cables for a while now, but Google might turn something up.
You could of course transfer very small amounts of songs from iTunes to iPod before the battery runs out, then re-charge via the wall charger, but this isn't an ideal solution.
EDIT.
Here you go, this is the type of cable I mean.
http://www.buy.com/retail/product.asp?sku=205878745&listingid=15805841&dcaid=179 02