I just got a 7100g from Cingular (i had a similar model from TMobile but changed jobs and went with Cingular this time around).

I'm unable to download any ringtones, and I can't download Themes! I'm actually stuck with the 1 Theme that is pre-loaded.

Can anyone advise?

Go to Rintones, Themes & Wallpaper. There is a Sticky Thread on downloading themes for a 7100. I have a 7100g and now have 7 themes. There is also a Sticky Thread for ringtones.

Cingular does not offer themes or ringtones for the 7100g at this time.

Using your 7100g's broswer you can download free ringtones from BlackBerry HELP!.

mftp.
As Gators86 said, look in the dedicated Ringtones, Themes and Wallpapers section on BBForums. Tons of information. Also check out http://mobile.blackberry.com