[91LX2Bfast]

I would go with an H or X-pipe. Now whether or not you want to run cats or not is going to be a choice you have to make. In my opinion, any of the major brands are going to be just fine, it just that some brands are higher quality then others, and then cost a bit more. Bassani, MAC, BBK, Magnaflow, Flowtech, etc.
The ideal setup would be longtubes into a short H or X. Short tube headers aren't really going to do much for power, but long tubes would add a very nice chunk of power to go along with that blower. The problem is you would need to get a new short H or X pipe with them, and then install would be very costly also. So that would shoot your leftover money budget to all hell.
So I guess I'll ask how far you plan on taking your Cobra?
If you want an reasonably affordable exhaust that will flow and sound great and make good power, then I say get an H or X pipe first, then catback.
If you want every last reliable HP, then I would say hold out till you can afford long tube headers and matching H or Xpipe. It won't be cheap, but thats going to be up to you. In the mean while you can get a catback to give you the sound you want.