A similarly configured Crud Pro discharges almost the same heat between a 2.8 and a 3.2. Certainly it's not something you can tell without an IR thermometer. The cooling profiles are also similar. So I'm doubting what the other guy said a little. Perhaps he's getting confused about the video cards vs the unit itself - but you really don't want the 2600 at this point in time. The other comment about the HDD caddies makes me wonder if the guy actually has a Pro. The design is elegant, but functionally it borders on the moronic as I have to reach around to the back of the drive and yank it out from there to get enough leverage without bending my fingernails.
The Pro is however one of the quietest dual-Xeons you can get and is acoustically inconspicuous in the quietest homes for the most part. This is mainly by dint of it being the worst cooled in the business. The mixing of the airflow between the GPU and the HDD is yet another bad idea - but Apple have different obsessions and people to cater for than people who actually expect genuinely reliable workstations. That would be me by the way. Which is why I spend far more time with my slightly noisier, but far better engineered for a world of real work, Dell Precisions.
If you're confident that you can track down the heatsink a couple of years down the line, then I'd say go Quad. Certainly you won't be using the power of the Octo most of the time.
But you know, the octo comes in handy for a variety of situations - say in something as simple as transcoding video. It can drastically cut times taken for that to happen. I'd say that if you're considering buying a Quad now and not waiting for a Nehalem-based Xeon, go for Octo, because the cost of that Xeon upgrade likely won't decrease in the future - it may even increase.
But personally, I would advise that if your G5 can stagger on for a couple more months, you're better off waiting for a Nehalem. It's not just the processor that gets an overhaul - the current Pro's 'regular joe' affordable GPU's are totally obsolete in the general marketplace for a start, and Apple's track record for offering upgrades in that respect is not good - so you're best off buying at the peak of a refresh. That, and subsystem upgrades on a dual-i7 Xeon board such as higher speed RAM, etc and the machine as a whole will be noticeably faster, for not a significant price increase over buying a Pro now.