I always advise caution when OCing books.  It is going to run hotter.  You may consider just drilling some vents in the plate to keep the plates intended use.  Have you tested temps?  You can use Temp Monitor for that, or not as I am unsure of compatibility on G3s.  You could use some Arctic Silver 5 on the GPU and CPU too which will drop a couple of degrees over the stock stuff (I have proven that it works).  Good luck and I hope your OC adventures go well.