If memory use is the problem, write the numbers to a file, call the program sort to sort the file, then read the median, 25th and 75th percentile from the appropriate line. Sure, this will take more time (sorting is N log N, finding the Nth element can be done in linear time), but sort knows how to deal with low memory (sort was written when 1Mb of memory was an awful lot, and out of reach for most).

Sorting 30m numbers took a couple of minutes on my aging box, but once sorted, you can quickly any percentile query.