Reinstall of OS after CPU Upgrade needed?

I recently upgraded my CPU from an Athlon II X2 250 to the Athlon II X4 610E(45W 4 cores -- can't beat the efficiency). Is an ubuntu reinstall required?

You shouldn't need to reinstall, Ubuntu is very, very accepting of hardware changes.

A good example of this was I have a Dell Latitude here that has a core two duo p9700 processor, nVidia Quadro graphics, two gigabytes of ram. That recently failed (my own fault) so to transfer the files off of the hard disk within it I put the hard disk into my other laptop, which is a Compaq Presario which has a Pentium dual core processor, three gb of ram, Intel graphics, a very different system. To my surprise it booted up from the old install on my Latitude with absolutely no driver issues at all. I was absolutely amazed. After I figured this out I tested it by hooking the hard disk up to my Desktop computer which contains an AMD Sempron single core processor along with nVidia Gforce 9600gt graphics card and four gb of ram and still it booted with absolutely no issues.

This has to be one of my favourite things about Ubuntu because it makes things so much easier.

You do not need to reinstall Ubuntu for new hardware being recognized. Just boot up after plugging the power on it and you're ready to go.

The ability to use multiple cores depends on the kernel and if it was compiled with the SMP feature (Symmetric Multi Processing). (Ubuntu overview of SMP) Since your last CPU was a dual core, it probably had the SMP kernel installed. Your new quad core will also use the same SMP kernel. (The SMP kernel will automatically adjust for the number of cores each time it is booted.)

To confirm that the kernel is able to see all four cores, check the /proc/cpuinfo file. For instance, a quad-core cpu should have four core lines:

dan@titan:~$ grep ^core /proc/cpuinfo
core id             : 0
core id             : 1
core id             : 2
core id             : 3

In general the nature of Linux is such that it starts each time as if it is loading on a fresh system (to put it simply). Unless you do something such as putting your hard drive in a new system you won't have to do a thing.