I've been searching forums and IRC logs, along with plugging numbers into a spread sheet to figure out the best current GPUs to buy for a dedicated mining rig.

But there are a few questions I just can answer without experience.

I'm basing my performance numbers on the data here: https://en.bitcoin.it/wiki/Mining_hardware_comparison

Based on that, I think I could build a system with 4x 5870s for ~$1600 total and get around 1.4Ghash. Those cards are going for like $215 each now.

That seems more cost effective than 2x 5970s since I'm having a hard time finding those for less than $650 each. If I had to pay $650 for those cards, I think the best I can do is around $1800 total for that system. I realize some cost savings by getting cheaper MB, case and power supply, but the GPUs cost 3x as much for 2x the performance!. And then I'm doing like 1.2Ghash?

I don't see how the pricing adds up unless I can get lucky and score some used 5970s on ebay for a better price. (I did just see one go for $470 + shipping.)

Unless the thing I'm missing is cooling. I haven't tried to build a 4 GPU system before. Is it going to be impossible to cool a 4x 5870 system that's running 24/7?

Also sound may end up being a consideration depending on where I end up keeping this.

Any advice would be appreciated.