Sounds fine, yes, the mobo, ram and CPU upgrade will give a lot, but weird, you cant get that E6600 higher.
That was my first CPU ever with OC on. I had it running 100 % stable at 3.6 Ghz with a CM Gemini II with 2 x 120 MM fans. It was great! I had an MSI P35 motherboard, so you also have a better motherboard. I just started with OC at that point, so I never tried higher clock, but I think I could. Have you read closer about OC of the E6600?