Open, and great question about upgrading / going top of the line. Who knows...but for purposes of this question, I wouldn't want to spend more than $1000 total.
You should easily be able to do something for under a grand. I would look for a pair of full bodied mics, but be sure to take into account all the accessories you'd need like mounts, cables, stand, and a preamp for your D50 to go XLR > 1/8". Other people know much more about this stuff than I do though.
I know you just bought that D50, but if you wanted to dip your toes into doing 4 track matrices (matrixes?) something simple like the Roland R-26 would get you there (with XLR inputs and phantom power for the full bodied mics) fairly inexpensively. They come around here every so often. But again, others know better than me.