Having problems with ORDER BY COUNT

[steed coptang]

Hi, I could use some help optimising a query if at all possible.

The query is below:

SELECT t_id, COUNT(*) AS cnt
FROM table
WHERE u_id
IN (

SELECT u_id
FROM table
WHERE t_id = $var
)
GROUP BY t_id
ORDER BY cnt DESC

The part that slows it all up is the "ORDER BY cnt DESC". When I run the query with the line it takes well over 0.5 seconds, without that line it completes in under 0.01 seconds.

I've got the following indexes:
u_t = t_id, u_id
u_id = u_id

Explain displays the following:
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY snatched index NULL u_t 8 NULL 145771 Using where; Using index; Using temporary; Using f...
2 DEPENDENT SUBQUERY snatched ref u_t,u_id u_t 8 const,func 1 Using where; Using index

I've tried changing the sort_buffer_size from 1M to 16M and also the read_buffer_size from 4M to 16M in my.cnf but they make no noticeable diffenence.

I can understand why it can't use the index for "ORDER BY cnt" because it is only using data from a part of the table. I'm just not sure what I can do from here to speed it up.

I'm using MyISAM and running mySQL 4.1 on FreeBSD.

Any help is appreciated, let me know if I've missed anything out.