The 400CN is developed just like standard color film. Just drop it off at the local minilab and you're done. Not knowing what the B&W 400 is, I can't comment on it, but if it's standard B&W film, WM will have to send it out, though some labs can handle it in house. What was the exact name of the 3pack?
How big an enlargement? That depends on the film, the camera (specifically the lens), the development, the enlarger, skill, etc. Also, depending on the subject matter and personal preference, grain isn't necessarily a bad thing. 8x10 should be easy, maybe 16x20 if everything's "right". It also depends on intended viewing distance. A slightly grainy 16x20 would look good viewed at a distance than a slightly less grainy print viewed closely.
Chris