InnoDB slow?

[Paul Gallier]

I'm running a large database which is currently using MyISAM.  There are 
approximately 300 million rows in about a dozen tables totaling 7GB of 
storage.  The system is averaging 257 querries per second, probably 
peaking at around 500-600+ during busy times.  We're running a single 
database with one programming doing insertions/updates and a web server 
doing only selects.  The problem is that the insertions/updates tend to 
bog down a bit when the web side gets busy.  I figured switching to 
InnoDB might help with the row-locking support, however, on my test 
system things ran very slow using InnoDB instead of MyISAM.  This was 
just running the script handling the inserts/updates with no web access, 
but a test that took 2 minutes 34 seconds using MyISAM tables took 10 
minutes 37 seconds using InnoDB.  I setup enough disk space to hold the 
tables and had seemingly sufficient ammounts of mmeory configured for 
InnoDB, so I cannot understand the drastic slowdown.  Any advise would 
be greatly appreciated.