There's really no good reason to buy a C2D now, especially since Apple is basically the only company using them (not for long we hope). But as for the standard i5 first gen and SB, the SB is pretty damn good. But if you can get a decent deal on a i5 than it's not a bad move. Of course C2D will do most of the daily needs, but just because it can doesn't mean you shouldn't just wait a little bit to get technology that isn't 3-4 years old when it's going to be at the same price point. At this point in time, I feel i5 or SB should be your only option.
I mean, I'm thinking about TV technology. Say you can get an older generation screen, I dunno DLP or something that can do everything you want (watch HDTV etc) or get a newer tech OLED (haha) or an LED tv that can also do the same thing, but at the same price point, why not get the LED screen over DLP.. even if DLP can do what you want it to. I'm not saying be a first adopter, but there's some good information backing up the SB cpus.