Unfortunately, Ruby 1.9.x is not copy-on-write friendly, mainly due to the garbage collection strategy. I would venture to guess that most Rails apps these days are running on some version of Ruby in the 1.9 series. So siong1987 is generally correct in the assumption that forking a Rails process n times will consume about n times the amount of memory as a single process. However, Ruby 2.0 has a new garbage collector (called bitmap-marking) that promises to be copy-on-write friendly, so as adoption of that increases your suggestions will become more important.
AFAIK, to use the brightbox packages on heroku you'd have to write a buildpack that either downloads all the dependencies and runs through the same build process that brightbox uses, or use something like heroku-buildpack-fakesu to create a fakeroot type of environment where you can install debs.
Another issue with using the brightbox packages is that if you happen to run into a bug, you'll have to figure out if the bug was caused by something non-standard in your ruby installation or if it's actually a bug in ruby.
One last thing to note is that it looks like the latest brightbox release targets patchlevel 327, while ruby core is at 392 (not counting 2.0.0-p0), so you're missing a lot bug fixes until the brightbox team gets around to building against the latest release.
