Hey, Rickity!
When I had Vista, I also used a 2.8 GHz CPU, and OFF was always running fine on it - I never had problems.
I guess what is most out of date for you now, is your CPU - not the OS.
3.2 GHz might be minimum, I'd say - more would be even better.
But that usually requires also a change of motherboard.
Which is always a big step - and a deep crater in the finances...