I have a rock of an old case that I really like. Problem is it has no more sides. Can I run it 'wide open' without issue or do I need the funneling(?) that a closed case creates with ? It's a P4 2.8 now and I want to go up to a Phenom.
As far as dust goes just keep an eye on it and blow it out if it needs it. By "no more sides" do you mean you just don't have them anymore? If you do have them and it just doesn't have places for case fans you can cut places yourself. Lots of old cases are like that.
Technically the case is not necessary. In general running with the sides off is fine and sometimes better than with them on, depending on the case and fan locations.
And the amount of meddling cats that you may or may not need to protect your rig from.