Yeah, I'd agree that if you only have a PSU exhausting the case, I'd definately be careful. My PSU runs at 800 RPM all the time, but it's inside the P182 lower chamber with a YL on 7V and two 1TB Seagates.
I would also consider trying to keep your HDD above the PSU in this situation, as I think that the GPU heat and CPU heat (if air cooling) will overheat your HDD.
If you aren't planning on moving it (water will add lots of weight to the case and I'm not sure polystyrene will hold it well for movements) or changing out parts, then by all means, water cool it. It will be a lot easier to cool with what you're trying to do.
Otherwise I'm sure you could come up with some, uh... rather imaginative... places to put a fan.
Remember, too, that the CPU isn't really the problem. I looked up some wattage reading a review of the new 45nm cpus, and I think that the quads used like 40-50 watts of power at load. This is even below a C2D, which is more than easily cooled unoverclocked passively with a Ninja, assuming some nearby case fans to help with minimal airflow.
So the big issue is really the internal heat and the GPU heat. I think that the GPU heat, compounded with Northbridge and VRM heat, flowing up into your hard drives and then through the PSU might be risky for your hard drives. Let's just say, I don't think I'd ever want my hard drives above my CPU and GPU. Heck, I don't really even like my CPU above my GPU, but that's the ATX format for you.
In short, what will you be doing with this PC? I'm assuming gaming with a 6800 and a quad, but you know what they say about assuming.