To expand on what Digby said, it simply makes sense to determine what conditions are most likely to cause the whole expression to fail. If you want to get very nit-picky, note that you check X first to make sure it is in range. Well, on a Pocket PC there are 33% "more" Y values than X. So assuming we're talking about square regions (on average), you should check Y first, as it has a higher probability of falling outside the range.
Dan East
