I'm testing out Google Apps for Business for my company, and I'd like to send mail both to the Google Apps MX servers as well as my existing server. This way I can test the functionality of Google Apps without disrupting my existing users and systems.
Is it as simple as adding the Google Apps MX records as additional MX records with higher cost (lower priority) than my existing MX records?
No. No matter how you setup the MX records, each email will only be delivered once (to the highest priority server that is available at the time of delivery.)