Currently i am running my server with a Single Quad Core Processor e5310 and the motherboard S5000PAL Supports another Processor.
I primary use this machine for Running my Web Apps (None of them are exposed to the Internet and hence very limited user load).
I am considering getting the second processor if it is going to make significanr difference. But i am not sure whether the four cores that i have are being used to their full potential as i don't do any video processing etc..
Also i am running 32-bit Ubuntu..I am planning to upgrade it to 64-bit server edition so that i can go beyond the 4gb memory limit.
Your OS should be able to report your current CPU load. If it is less than 50% loaded, you won't see any significant improvement by adding another processor.