Usually what dell says is what you get, but the problem is that you can't be too sure. Even if they say M745A you might end up with a M745, so you never know, it doesn't happen, but it's impossible to predict.
But I think you should just call them up, its better... not necessarily buy it on the phone, but just call them up and ask them what FSB the computer has, and whether they are going to ship what they say.