Hi Josh!
The 400D is a great camera. It's a shame you haven't got a longer lens or telescope to take pictures through, but you'll be able to do some nice widefield stuff of the eclipse and the s3is will do some nice up-close stuff.
For practise, I'd suggest photographing the moon in the few days leading up to the eclipse - the moon won't be full but it will still be good practise to make sure you have the right settings.
Read the article linked here:
http://www.iceinspace.com.au/index.p...63,409,0,0,1,0 which has the timings and altitude of the various stages of the eclipse from Brisbane.
Good luck and I look forward to seeing your results!