Mal, your best bet is check out here:
6.7L Power Stroke Diesel - Ford Truck Enthusiasts Forums

Depending on how much coin you want to throw at it, you may want to get a programmer and EGR/DPF/Urea injection delete, should improve the fuel mileage a bit, and give you up to 200 more hp, and you don't need to worry about filling up with Urea when you get it serviced - and its more conducive to running on crap quality fuel.

Other than that, other modifications are limited more to the interior/exterior, leveling kits/bar work/lights, better shocks etc.

The 6.7 are great trucks though. I am thinking of getting one myself to replace my current F350 (I just wish they still made them with a manual transmission!)