There's basically no difference between "companies."
The cards are same, and the difference is the price, warranty, and bundle.
Decide on the one that you think will be best for you. If you like a whole bunch of bonus software, or someone has a better warranty, decide on that.
Yes, you can run nVidia cards on ATI based mobos, and vice-versa.
You can also run more than one, unless you are trying to use SLI or Crossfire, then you would need the matching motherboard chipset.
Xbit Labs said possibly late June '09.
http://www.xbitlabs.com/news/video/display/20081015152651_ATI_RV870_and_Nvidia_GT216_Expected_to_Arrive_by_Late_June_2009.html
If I were you, I would save that $200, for a future upgrade.
If you really want to upgrade, I agree with the suggestion for a used 4870 or so. EBay should have some.
Have both. You never know what will try to get through and whatnot.
I would say a router firewall is better, but having Windows Firewall and router firewall is best combination IMO.
EDIT:
If I was to choose just one, the I'd go for the router firewall.