I agree with the posts above. I started with a cat back 4" system and then got a front pipe and split dump. I didn't notice a massive HP gain, but it will come on boost quicker, and increase top end.
As for doing the dump/front first - if you're looking for most HP gain, then the cat back will be better value for money to start with; then do the dump/front pipe. That said, the closer the restriction to the turbo, the greater the effect on flow efficiency; for example, if i put a silencer in my rear muffler (reducing the effective diameter to say 2"), it will have less of a flow distubance and thus effect than having a 2" dump pipe and 4" cat back.
So, in conclusion, do the cat back first, then worry about the dump/front if you are strapped for the mulla. Bang for buck.