For a load with more writes and reads, you will benefit from InnoDB. Because InnoDB provides row-locking rather than table-locking, your SELECTs can be concurrent, not just with each other but also with many INSERTs. However, unless you are intending to use SQL transactions, set the InnoDB commit flush to 2 (innodb_flush_log_at_trx_commit). This gives you back a lot of raw performance that you would otherwise lose when moving tables from MyISAM to InnoDB.
Also, consider adding replication. This gives you some read scaling and since you stated your reads don't have to be up-to-date, you can let the replication fall behind a little. Just be sure that it can catch up under anything but the heaviest traffic or it will always be behind and will never catch up. If you go this way, however, I strongly recommend you isolate reading from the slaves and replication lag management to your database handler. It is so much simpler if the application code does not know about this.
Finally, be aware of different table loads. You will not have the same read/write ratio on all tables. Some smaller tables with near 100% reads could afford to stay MyISAM. Likewise, if you have some tables that are near 100% write, you may benefit from INSERT DELAYED, but that is only supported in MyISAM (the DELAYED clause is ignored for an InnoDB table).
But benchmark to be sure.
