That's a tough one. I think I'd go with the incremental approach: M60s now, save money for a good sub next, then do a center, and then do surrounds. If you've already got something covering surround duty (even if it's not great--I don't know, I've never heard Newtons), then you're ok there. You'll want the sub before a center, I think.

I am the Doctor, and THIS... is my SPOON!