MySQL Partitioning Performance

I have a MyISAM table with billions of rows that is still causing me problems because the indexes, even after I shortened them as much as possible, do not fit in my 64GB of RAM.

I am unable to index the table and this is causing serious problems.

I thought that partitioning might be the answer to the problem, but I'm not sure because I SELECT on all kinds of different column combinations.

The table structure is:

CREATE TABLE research_storage1b (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `word1` mediumint(8) unsigned NOT NULL,
  `word2` mediumint(8) unsigned NOT NULL,
  `origyear` smallint(5) unsigned NOT NULL,
  `cat` tinyint(3) unsigned NOT NULL,
  `pibn` int(10) unsigned NOT NULL,
  `page` smallint(5) unsigned NOT NULL,
  `pos` smallint(5) unsigned NOT NULL,
  `num` tinyint(3) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  KEY `pibnpage` (`pibn`,`page`),
  KEY `word21pibn` (`word2`,`word1`,`pibn`),
  KEY `word12num` (`word1`,`word2`,`num`),
  KEY `year1` (`origyear`,`word1`),
  KEY `catyear` (`cat`,`origyear`),
  KEY `pibn` (`pibn`,`word1`)
) ENGINE=MyISAM DEFAULT CHARSET=ascii COLLATE=ascii_bin
DATA DIRECTORY='/storage/researchdb' 
INDEX DIRECTORY='/storage/researchdb';

The indexes are representative of the different SELECT statements, i.e. sometimes I'm retrieving pibn based on the word1, and sometimes I'm retrieving word1 based on pibn.

If I had to choose something to split the table on, it would probably be word1 because this is most searched for. But there are some queries that will be selecting word1, or counting word1, in which case would I be correct in assuming the partitioning would drastically slow those queries down? There are around 500,000 unique values for word1.

My main questions are:

1. Should I use partitioning (will it solve my indexing issue)?
2. Should I partition on word1?
3. How many partitions should I have? (I'm thinking maybe 100?)
4. How do I modify the CREATE TABLE to have 100 (or however many) partitions split on word1 (or another column)?
5. Since word1 is not a primary key... do I have to change the primary key to an index, or something like that? (I didn't fully understand this part of the partitioning docs/tutorials.)

Please help if you can, thank you.