In terms of memory usage it's generally going to be Python > Ruby > PHP, which of course leads to Django > Rails > CakePHP. Not just memory but that also tends to hold for raw performance. EDIT: Also worth noting that there are, of course, no absolutes here. There are plenty of usage scenarios in which Ruby will beat Python, hands down. I think we can all agree that Ruby and Python will always beat PHP, though :)
Here's a straight-forward 3-way benchmarking (with Symfony on the PHP side of things) that bears out the above: http://wiki.rubyonrails.com/rails/pages/Framework+Performance. Though of course it's easy to find stats to support your own viewpoint :)
That said, it's still very easy to make a crappy, slow, and inefficient Django application and a lean, fast, and efficient Rails application, or vice-versa. Skill, knowledge, and expertise with the system you are using will do far more for its memory and performance footprint than just the framework itself. Database optimizations, server choices and architectures (Apache vs. proxy setups using nginx/lighttpd, etc.), and fundamental design decisions are likely going to overwhelm the framework's inherent characteristics pretty quickly.
So I guess what I'm saying is if your team knows Rails, and your expertise lies in Rails, I would stick with Rails.
EDIT: Some more recent Ruby v. Python v. whatever benchmarks. Not framework specific but potentially useful: