Interested in what you would use for a $450 build cost SFF biz computer? Has to handle line of business application for dental office that uses digital imaging. (The current machines are Core 2 6400 w/ 1GB ram and do ok).
For a CPU you could get the Intel E5200, it's slightly faster than the E6400 you have now, but what's more important: it uses less power and generates less heat, that's very important for a SFF computer.
Get 2GB of RAM (2 sticks of 1GB, not one of 2GB): it makes your machines noticeably more responsive, especially in Windows Vista.