With some experimentation, I've been able to determine that Ubuntu 6.06 LTS (the Dapper Drake) boots fine on a virtual machine allocated up to 588MB of memory. Any more than that (f.e. 592MB), and it stalls for a while "loading hardware drivers" and then exhibits a variety of problems.
Installing Ubuntu's i686-specific kernel didn't seem to help matters, nor did upgrading to the latest beta version of Parallels (build 1862) appear to make any difference.
Installing Ubuntu's i686-specific kernel didn't seem to help matters, nor did upgrading to the latest beta version of Parallels (build 1862) appear to make any difference.