If you are willing to spend a little more the Intel E7200 is significantly faster than the E2180 or 5400+ without overclocking.
Newegg.com - Intel Core 2 Duo E7200 Wolfdale 2.53GHz 3MB L2 Cache LGA 775 65W Dual-Core Processor - Processors - Desktops
Otherwise the 5400+ is probably your best option.