One of the MYD files is 100 gb and another is 40 gb. For both these tables, the indexes were defined before any data was put in. The tables were then populated using load data infile and a trigger to create the geometry object. As such, I can't really tell how long the index creation took, as it was all part of the overall load process, which did take 3 days on one of the tables. It is likely that the index creation would have taken a very long time, had I done it after data load, as MyISAM tables are completely rewritten in the process.
Now that these tables and indexes are set up, I can do several thousand inserts a second, of fairly complex polygonal data, which I consider acceptable.
My config file for sort buffer size, etc, is more or less the same as the sample huge.ini that comes with MySQL, as the box in question, is as 64-bit beast that only runs MySQL.