Try googling for `two dimensional knapsack'. That gives some promising looking hits.

For the non-CS types in the audience, this type of problem is known as `the knapsack problem' because it's analagous to finding the largest amount of fixed sized objects weighing w_i units each that can fit in a bag that can hold x units.

Update: Glancing over some of the google results seems to indicate you've picked a doozy. Having a few fixed layouts for the groupings you're most likely to use probably would be the quicker solution.

Oh, and: Post #300. Wh00t.