I too own a 2008 Mac Pro and replaced my aging Radeon 2600 with this puppy. First thing I noticed out of the box: it is HUGE compared to ATI and NVidia's older GPUs from a couple years ago. To the tune of 2x as wide and 2.5x as long. It also requires that you plug it into one of the 6 pin power adapters on your motherboard, which isn't hard but if you're not used to doing stuff like that it might be a little scary at first. Because of both of these factors I was worried it was going to be much noisier than my 2600 (drawing more power, larger fan) but ironically the opposite is true. This card is much quieter when booting up and makes no "extra" noise that I've heard (yet) during regular operations. I will be using this with the Adobe suite and some other GPU-intensive apps, will report back if I begin to hear any real noise issues.
The best part about the card is its dual Mini DisplayPorts, but unfortunately because Apple doesn't make a Mini DisplayPort to DisplayPort cable, you have to use 3rd party solutions to connect with a more expensive LCD for color accurate work, and it seems some of those monitors (NEC and Eizo) are not compatible with some of those cables (StarTech, maybe others). A lot of folks have been experiencing problems with this, so just expect that if you have one of those monitors, you may be relegated to using your DVI connection until the monitor manufacturers and/or cable companies can discover the cause of the problem and solve it.
Haven't tested with any hard-core games but benchmarks I've seen indicate it's a nice boost over the Radeon 4870, the best upgrade kit available before this one.
Update: For gamer types, I've been playing COD 4 regularly on high res/settings for a while now, and the card handles it easily and does not heat up / make any racket during play. This is probably the best GPU I've ever owned on Mac. Very happy with purchase.