Connect to the Linux server using “Go > Connect to Server”

From a few minutes of searching, it looks like Ubuntu 11.04 would be a good fit and match most of your requests, and what you were looking to use as well.

64bit would be needed to reach more than 4GB of RAM. And there is a 64 bit version, although (to me) confusingly named AMD64. This would be the version you need.

There are also Magic Trackpad drivers available, and a good writeup available on the Ubuntu Wiki.

I also found a good site documenting installing 11.04 on a MacBook Pro. This should be rather similar to your instal.

Finally, in most cases, choosing the right Linux distro is also a feel and how you like the install. But in accordance with your list of support, it looks like Ubuntu will cover this.

The BIOS emulation only kicks in if you use MBR or hybrid GUID + MBR partitioning. By default, Macs only use GUID partitioning. Installing Windows via Boot Camp enables this hybrid partitioning, but you can in principle also do it manually for Linux. The downside is that you then have to maintain two partition tables and make sure to keep them in sync.

It is rumoured that you can still boot Mac OS X from a pure MBR, too. I've never tried this, and even if it does work, I suspect things like firmware updates and FileVault 2 FDE will not work in such a configuration. You'll probably also have problems with multi-booting unless your bootloader supports OSX directly.