I also agree that the difference in braking power lost by moving your pads down 4mm will be minimal.
So,
if you switch to 700c wheels (I'm on record above as advising against it) use your present brakes for the moment and see how they do.
If you feel you're limited on stopping power, then switch to dual-pivot brakes.
What sets dual-pivot brakes apart is simply that they have greater mechanical advantage compared to single-pivot brakes. This has the downside of meaning that pads on dual-pivot brakes start closer to the rim already (when you're not braking) so there's less room to play if you bust a spoke and want to ride home.
Seeing a picture of the brake won't give you more of an idea of what's going on here.
But if you want to look, here is Nashbar's house-branded Tektro long-reach brake:
http://www.nashbar.com/bikes/Product...1_10000_201527
There are also even longer-reach versions of the Tektro brakes (originally-commissions by Rivendell)
http://www.rivbike.com/products/list/brakes