Most boards that can host the E2140 processor support 2GB DDR2 DIMMs and will typically support 4GB (if only two DIMM sockets) or 8GB (if four DIMM sockets) system configurations. If your board has four DIMM sockets, you can add either two more 1GB DIMMs (for a total of 4GB) or two 2GB DIMMs (for a total of 6GB). The 1GB DIMMs must be in a separate memory channel from the 2GB DIMMs.
I gotta say, this is a really, really old system (11+ years). Compatibility and hardware stability issues are (or will soon be) occurring. In my mind, spending anything on this system is throwing good money for bad. You really should spend it on a more modern system.
Hope this helps,