With the chain set, how do the numbers equate to difference in speed
The numbers don't equate to speed; that's all down to the legs I'm afraid!
If your riding is likely to take in steep climbs or long descents I'd recommend a 50/34 or similar. If it's flatter, or lots of little hills, then a 46/36 will prob be fine. 50/34 offers more gears at the top and bottom ends, but with the downside of bigger jumps between some gears whereas 46/36 = less lower gears for uphills and you'll run out of gears sooner when bombing along on the flat/downhill, but the ratios are closer therefore can be smoother going through the cassette.
CX bikes tend to come specced with 46/36 as they're designed for going quickly off road rather than up and down big hills.
I looked at lots of CX bikes (and am still after one now) but went for a relaxed geometry road bike in the end partly because of this gearing issue. It's flat where I live so 46/36 would be fine for my commute, but I knew I'd be doing lots of long rides in the Peaks, Dales, Wolds etc too, so a 46/36 would have been no good to me there. Hence I went for a road bike, which copes just fine with canal paths incidentally.