If you're looking to configure mail from your slicehost and you do go with the Google Apps option (that's what I use), you might like to have a look at this guide to getting sSMTP working with Google:
I can't recommend sSMTP enough to anyone that doesn't want to maintain a mail server. All UNIX systems should be able to send mail and this is very much still the case on modern Linux systems, but this is often overlooked.