It appears that Mittens has taken the Maryland and Washington, D.C. primaries, and is on his way to win the Wisconsin primary. Hence, the delegate count is now:
Romney 663
Santorum 256
Gingrich 140
Paul 67
The rest of the April primaries are as follows:
April 21
Missouri 52 delegates Non-Binding Caucus
April 24
New York 95 delegates - Proportional Primary
Pennsylvania 72 - Direct Election
Connecticut 28 - Hybrid Primary
Rhode Island 19 - Proportional Primary
Delaware 17 - Winner Take All Primary
Since Missouri is a non-binding caucus, it's of less importance than the others. I suspect Santorum may be able to blunt Mitt's gains by taking some of the New York delegates , and perhaps, as a native son, all of Pennsylvania's.
In May, Santorum should be able to gain a bit on Mittens. On May 8, Indiana and wet Virginia may give him 77 delegates, while on May 15 he could win a few in Oregon. On May 22 he could pick up most of Kentucky's 45 delegates and most of Arkansas' 36. On May 29, assuming it doesn't support Ron Paul as a native son, Texas could give most of its 155 delegates to Santorum
Indiana May 8 -- 46 delegates - - - - Hybrid Primary/Caucus1
West Virginia May 8 31 - - - - Direct Election
Nebraska May 15 -- 35 delegates - - - - Non-Binding Primary
Oregon May 15 28 - - - - Proportional Primary
Kentucky May 22 45 - - - - Proportional Primary
Arkansas May 22 36 - - - - Proportional Primary
Texas May 29 155 - - - - Proportional Primary
All of this means that Romney will maintain his substantial lead, but probably won't hit the magic number in May. That leaves June. On June 5, in winner-take-all primaries, California (172 delegates) and New Jersey (50 delegates) should give Mittens 222 delegates. Added to the 663 delegates he already has, these would give him 885. Added to those he will have already picked up in New York, he may well have the magic number by the time of the convention.