Well I have the value set at 128 meaning 128meg. Exactly what to set it too is a bit up in the air because bioware said in there official testing that values over 64 had little to no affect, but they said this was a test ran on the single player campaingn and custom modules may differ, they didnt say why though. So if anyone can think of a reason why it would that would be helpfull. Yes you have to add that command line in, just add it to the top of the nwnplayer.ini file under game options, dont include quotes.
So somewhere between 64 and 128 should do the trick. Try 64 and see if performance improves. I dont crash all that much anyway but since changing it a few days ago I have only crashed once, I cant say for certain this is giving me any benefit yet though.