The Accelero S1 is a bit of a pain to install, but so far so good. First the install (X1900 XT):
Remove stock fan assembly (8 screws.)
Stick on RAM heat sinks - this looks easy, but they didn't want to stay stuck long enough to complete installation. After they heat up, the adhesive / heat conductive material should harden, but it is getting them to stick enough to complet the installation that is the problem. Finally discovered that smearing a fraction of a drop of denatured alcohol on the heat sink surface softens it enough to really make them stick.
Install the huge passive cooler - easy.
I've checked all temps (Hardware Monitor ver. 4.2) before and after the install (BTW, this X1900XT is replacing the 7300).
The Northbridge Heat Sink is the one I expected to see the biggest increase in (proximity) as well as the hard drive (located directly above). The Northbridge is running (on average) 1 degree C. warmer, and hard drives are also 1 degree C. warmer. No other temps (memory, cpu, power supply, etc.) show any difference than before.
Pretty darn happy with the S1, and exactly as quiet as my old, weak 7300.
I'm not a gamer, but the card has been working out with Aperture and FCP, and definately shows smoother video than the 7300 did (which wasn't really bad.)
I got the non-Mac X1900 XT PCIe used from eBay, bought the 4500 power cable ($30) and flashed the ROM for the Mac, and it seems to work perfectly - for half the price of a new Apple ATI product.
John F