What's the purpose of 2 250GB drives and then one 500GB one?
Do you have the Ultra 120 or the Extreme versoin? (Just curious)
When you OC, use a 1:2 FSB:RAM ratio. If you use a 1:1 ratio and want to OC your memory, you'll need a FSB of over 533MHz!
How far do you plan on OCing?
Your cooling setup sounds OK. I'd use the Scythe fan on the Ultra 120 instead of the Thermaltake one. I doubt your 8800GTS needs additional cooling, and the Scythe fan is probably quieter. Can't help on the top vent question. I'd just try it with and without, and see what happens.