I've owned my BFX for a short time but have put a fair amount of money into it and it's not done yet...
First off, buy these. Fit them in the axles and in the gearbox, everything will run alot smoother and be better equiped to cope with a power hike.
Then I bought this. The power difference was insane, its so so sooo fast now. The only problem is, it's such an animal that its hard to control and I've broken a few bits and pieces on it! I'm on ebay ordering an aluminium suspension set up as I type this!
Remember the chassis is the same as the Wild Dagger and Twin Detonator so search for parts using these names as well. There seems to be many more parts made for the Wild Dagger than both the BFX and TD put together!
I also bought a set of these to make the truck a little more stable at speed, it was a wise purchase but they take time to set up to get the weight balence correct.