The gigabit upgrade gives an impressive boost in performance, especially if you are transfering large amounts of data between a Macola server and another server running ODBC.
Dual servers don't do much for Macola but they do allow the operating system to offload tasks like print and file service to the extra processor, so if your site has invoice printers, etc hooked directly to the Macola server or if the server is hosting file archives for other systems, you'll see a boost.
Biggest bang seems to come from memory upgrades. Macola loves RAM.