Thought maybe this may be a good time to bring this up again.
Probly the best bang for the buck mod is the Wix 6637 air filter. I removed the air box top and some minor modification to the lower box to fit the filter. You can go a little more than this and add some air movement stuff.
Then I would add a gauge set, I found a BD X-monitor 3 in 1 digital gauge pod with limit alarms, for $200 on ebay. Easy to install and will get your attention if you over boost, over pyro, or heat up your auto trans.
Next on my list would be a tuner, I prefer DP Tuner. This custom programmed chip is an awsome improvement to the Ford powerstroke engine. Having the air flow from the 6637 really makes this the engine go and can get you better mpg's (if you say out of the throttle). This would be a less than $500 mod, depending on the tunes you get.
Of course, with all that air going in, it needs to go out. 5" mandrel bent exhaust kit. I bought my first one from Mark in Idaho. Was about $300 and sounded nice. I noticed lower EGT's after the larger exhaust was put on. Besides the nice sound, it added nice looks and got me plenty of comments.
Please, if anybody else has free or low cost mods, add to this list. I know there was a lot of talk about tank mods, cackle cure, wastegate modifications and more.
Filter cover for the 6637 from Pete ($20)
CCV mod ($20)
Clean CAC boots, tighten clamps (free)
wastegate delete mod (free)
torque rocker arm/injector hold-downs to spec (free if you have a tq wrench)
GPR led mod ($10)
Scangauge (not necessary, but very cool $220)