The minimum memory requirement for Ubuntu 7.10 is 384MB of memory for desktop CDs, and 256MB for other installation methods. (Note that some of your system's memory may be unavailable due to being used for the graphics card.)
The minimum memory requirement for Ubuntu 7.04 is 256MB of memory. With only the minimum amount of memory available, the installation process will take longer than normal, but will complete successfully, and the system will perform adequately once installed.
http://www.ubuntu.com/getubuntu/releasenotes/704