Problem:
Your implementation of the check for existence of Imagemagick on the running serversystem can cause "500 Internal server error" messages on some systems where open_basedir is in use, especially shared-webhosts. This happens in the current stable 3.0.2, RC8 and SVN as well.

Solution:
Change line 401 of install/install_install.php (in 3.0.2 => may differ in unstable versions) to:
if (@realpath($location)!=false && @is_readable($location . 'mogrify' . $exe) && @filesize($location . 'mogrify' . $exe) > 3000)

Works fine for me and should not cause any other problems.

The trick is that realpath($location) cannot cause error 500 as is_readable would do when you try to access paths outside the defined open_basedir directories.