If you are adding a second computer
1. Do NOT add a second monitor. Monitors are huge consumers of power. Flatscreens are very efficient however, there is no need to add either a monitor or flatscreen. Use a KVM and/or Remote Desktop and/or VNC
2. For the computer, the biggest difference is the PSU ... Active PFC and other new technologies contribute to higher efficiency. There are some new power supplies with > 85% efficiencies. Most current and older PSU designs have efficiencies in the under 60% and under range. The higher efficiency PSU's are pricier but pay me now or pay me later again and again.
3. Use a low end video card for a cruncher. Recent video cards consume huge amounts of power.
4. Consider a dual core or dual processor machine ... One power supply, one video card ... twice the processing power ...
mondobyte