The main thing about overclocking my CPU is knowing what temperatures are fine. But if it can take 45C then I should be able to do it fairly easily as it's running a little over 30C at stock settings.
As for cases, I have got a
Coolermaster 690. Had quite a lot of room to work in, and it was a good price at the time. (~£50) But I can't see it being any bigger than the Antec 900, and if that's not big enough then I guess you'll have to keep looking. The other case I considered, but didn't get due to the price, is the
Antec P182. At £80-£90 it gets quite close to you other option, but has some amazing reviews. Quiet and well cooled.