Polyphonic ringtone file types

I just bought a Nextel i730 phone (Nextel phones are all made by motorola). I would like to put new ringtones on it, but I don’t want to pay for the WAP services on my phone. I have access to a cable to connect my phone to a USB port, but I don’t know what file type my ringtones should be or how to put them on the phone? Is it standard MIDI or something else? Once I get files in the right format, how do I move them to the phone?

They’re just standard midis.
I recently purchased a phone-to-USB data cable for my siemens. It came with software that allowed me to access the file system on my phone, and transfer files across as you would normally do with removable devices.
Software should’ve come with the USB cable that will allow you to do this.

I didn’t buy the cable, a friend loaned it to me… I will ask him about software. In case he lost it, is there any third party software available that might work?

Looks like myJAL will let you upload ringtones (either MIDI or WAV) or Java applets.

Along the same lines, what format should wallpapers be in? JPEG? BMP?

pretty much all of them, your phone manual should tell you the types it supports.