1. Does it or does it not work with Vista x64 and 4gb+ RAM?
It will work, but you do need the updated beta drivers if you're using Media Center as outline on their Vista page;http://www.hauppauge.com/site/support/vista.html
If you use 3rd party software like Myth TV you tend to be better off for both companies.
I'm just about to add a Hauppauge 1500 to the ATI650 I already have. I MO it's more about personal preference than other things, and to me form factor is more important since both companies do a real good job.
And as for random noman's post, both ATi and Hauppauge have their issues, and both struggled with Vista early on (more of a vista issue than ATi of Hauppauge's), however they are both much better than the other options out there and have improved alot over the past year.