Question: Can I increase the number of processors dedicated to my Ubuntu Server 10.04 virtual machine without creating problems to its functions?

It will be accepted without hassle or I should do something at reboot?

If I take a snapshot before that can I be sure to get back to my working 1 cpu configuration?

Thanks,
D.

Answer: 
Go ahead and add the extra CPUs, Ubuntu happily accepts them. Just reboot your Ubuntu server after adding the CPUs.

Please be aware that depending on your application the extra virtual CPUs might not actually help you a lot. Usually yes, but there are exceptions, so please test and benchmark your application extensively after CPU upgrade.
One thing you have to watch out for is that ESXi will not schedule your VM until it can provide all the vCPUsthat you assigned to it. This may lead to your VM having to wait for vCPU resources to become available which man not be what you want.