ok, 4850 is installed and I'm mostly happy with it. Here's the pros/cons list:
pros:
1. fan is no problemo at all. I read taltamir's 4850 guide in the video section and made a custom profile. I run folding@home 24/7 and running the fan on 37% is perfect for anything other than furmark with the card oc'd to 690/1075.
2. heat isn't an issue. card idles at 54c now and loads with folding@home at 74c.
3. AA/AF - had problems using this at all with the 3870, now it's a piece of cake 🙂
cons:
1. have an older bios, so card idles at 500/750. not a major issue for me b/c it's always in use, but could become an issue later if I stop.