What message do you get when WoW crashes? It might be very worth it to just copy + past your WoW folders and try again using the new copy. This worked for me before, only takes a few minutes, and it's free too

I'm running 5 WoW's on a system just a little bit faster then yours, but with 4gb of memory.

Btw, I use keyclone's maximizer to set my WoW's, don't know if it matters