SubGit will run on the most modern systems. Requirements for the server are:
SubGit 2.0 uses native pre-compiled executables for hook script, so that there is no overhead on starting JVM for each push (assuming background process is already running). Some of the more exotic systems might not be able to execute pre-compiled binaries, then SubGit falls back to the Java-based hooks. In this case memory requirement is increased by N*32Mb where N is maximum number of concurrent pushes.
Memory available consists of physical memory and swap, so memory requirement gives an idea of how much memory might be allocated by SubGit at peak time, it does not mean that server must have that much physical memory installed.