NSR Cams would be nice you can go with FRPP Hot Rod Cams, or Detroit Rocker cams or Comp Cams XFI Stage 1 or Stage 2 cams (depends on what gears you have 3.73s or higher the stage 2 cams would be the better bet in my opinion) or Mother Thumper cams but they are mostly for sound from what I hear. That may be about$ 800 max usually for the cams and a decent set of Long Tubes can range from around $700-1500. Obviously get your tune redone and I would suggest getting a Dyno tune to get the most out of your setup
i know they won't make your car sound better but as far as beefing it up and making it faster from what I've read getting new gears is the best thing you can do
[ame=http://www.youtube.com/watch?v=e_Ftc82mT7I&feature=mfu_in_order&list=UL]Mustang 2007 Pypes mid muffler setup - YouTube[/ame]
I love this setup but its like $1300
What year is your car. I had an 04 Gt and i put stage 1 Crower cams in it. I had more power up top but overall i was not very happy. In hindsight I wish I had gone with the stage 2 cams.
I'm also gonna throw in another vote here for new gears. Unfortunately they don't improve sound but it is probably the cheapest and quickest way to add a crapload of power.