It's not going to be backward compatible with PS3 games on disc, so I wouldn't stock up yet. If it does let you play PS3 games it will be via the streaming service. Perhaps there will be a way to unlock the ability to play a game by placing its disc in the drive, but save your pennies for now.
I looked at the Killzone screen shots on Flickr, they're original resolution was 3840x2160, so there's a good chance the PS4 does 4k output.
I'm also betting the price will be under $300 at launch. It's not that powerful of a machine, and with today's gaming market no one is going to pay $600 for a home console.
I will likely buy one, but the lack of power is making me think about building a gaming PC. I've not seriously gamed on a PC since 1996 (basically when I started working on PCs for a living, and tuning/troubleshooting a machine to just play a game felt too much like work). Though, the Steam Box thingy looks interesting, I might go that way.
Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
Sony PS3, surround backs