You nailed it. There is not a huge difference between x38 and x48. A few frames per second in crossfire on most games. The x48 will overclock a bit better.

The same can be said for P45. If you are gaming on a 22" monitor or smaller, the reduction in frames per second is minor, compared to x38.

While your PSU may be top notch, it's going to have to work pretty hard. It will do it, unless it's already old, but you'll be speeding up it's life cycle.

I would consider the PC P&C 610W or the Corsair 650TX to be good choices for a new single 4870 build. If you add a second, then you'll want a really exceptional 750 or more likely a quality 800+.