I wouldn't buy a chip for these cars. They don't take chips, they have a flashable ECM and TCM. I recently started with HP Tuners, but that gives you control of a lot of stuff and 0 guidance other than their forums. If you are looking for something pre-canned, I've heard the Diablo tunes aren't finished for the 07s or 08s yet. That leaves Superchips Cortex and Vector that I know of... maybe some others know of some different tuners. But they all come as something that plugs into the diagnostic port under the dash.
