The cache size does make a difference. I'm going to get a 1GB module soon.
Somebody else has a similar problem to yours: hardware - Incredible low disk performance on HP DL385 G7 - Server Fault
As far as controller lifespan goes, I've been recycling old hardware for as long as I can remember (in non-production environments). I don't think there would be a problem with a used controller. Batteries might go bad but you can refurb them on your own. If this is production I would get all new components.
In regards to your cables, the P400 cables won't work on the P410 controller and neither will the P400 battery. It's a shame the P400 is a dime a dozen. It would be nice to recoup some $$$.