Ok, usually I start the miner on slush's server with regular settings, just not specifying the -w value.
I read about it somewhere else, so I decided to mess around with it.
I'm running a Nvidia GeForce 8600M GT, and generally get about 5000 khash/s without specifying the -w value.
Then, when I set -w to 512, I got a hash rate of 1880356953 khash/s! I'm pretty sure that's messed up. And it ran like that for about 30 seconds before it crashed.
Glitch....or what?
On win7, btw.