I have a 99 GS400 stock. I am looking to get a little more horse power out of her. Should i change the headers and exhaust? And how much should this cost?

 

Buy the SRT intake/ecu, or some other intake and a SAFC-type device to tune it with. Easy to install and uninstall yourself and relatively cheap. Headers will be worthwhile, but will cost a lot more and if you cannot install them yourself, they are costly to install/uninstall.

How much more power do you want, and how much are you willing to spend?