How well does the built-in GPU in a Intel Core i5 2500K support unity? Will I need a discreet graphics to run it?
Even tough there are many people running Unity3D around with the builtin GPU's from Intel the results can be a bit slow or not as snappy as with a discrete card.
Some people have no complaints over using it but if you feel it is not as fast as you would like then you can always try Unity2D, which will mostly not care what card you are using.
Unity3D's compiz takes a heavy toll in the system's performance and if not compensated with a fast GPU it can feel slow. (It can feel slow even with a dedicated GPU).
Your GPU's driver come directly from Ubuntu repo's and should be installed by default when you install Ubuntu 11.10.
You certainly do not need a discrete graphics card to run Unity. I have a laptop with a Core i5-2520M (2.5 GHz, 3 MB cache, dual core) and it runs Unity perfectly.
Unity does use the functionality of the GPU but it has much less demands than a 3D graphics heavy game; you do not need the latest and fastest graphics card for it.