matthew: While I agree with most of the analysis above, there are a few (minor?) inaccuracies:
It would be great if LHR-JFK ran 5000 miles. Actually it's 3466.
Droneklax's advice about the difference between "H" and "B" is well-taken. Right now, r.t. NYC-LHR in "H" is around $575, "B" is about $800. During the summer, the numbers are usually around $1050 and $1250. The ascending price sequence, from "H" up, is: H,M,B,Y. "H" AND "M" are 20K each way for the upgrade, "B" and "Y" are 10K each way. So "M" does you no good, unless the other differences in the "M" fare, at the time, over "H" (such as minimum stay & advance purchase) are useful.
Along with the others, my recommendation is to buy the ticket and upgrade with miles whenever possible, until and unless you reach the point where you're already 1K and the status miles are no longer useful. In my own case, having just extended myself to make PE for the first time in 3 years, I'm going to continue pressing until I make 1K (at least, that's my plan for 2001), since the additional benefits are considerable. Every time I cash in a free ticket to go JFK-LHR in "C", that's 6932 miles towards status I just gave up.