Qube, I know you are excited about the 540, but please slow down and consider what you are buying. Please create a plan of what you want your car to be and why you want to get something.
Further knowledge or research or learning will tell you the limitations with OBDII. There is power to gain, but not as much as with an OBDI. I have the Dinan stg II and it is good for raising the rev limiter a little. a slightly quicker throttle response ('99+) and it is suppose to compliment an intake. Be very careful with choices for the intake as K&N types will mess with the MAF from the oil and others that stay in the engine bay will ingest hot air. Power is good. On my last post about the e39 I described what you whould do first. You already have a 4.4L V8 so the extra money should go to the suspension, brakes, tires. Thats it. Don't mess with the software as you need some other upgrades before that needs to be changed. You need to harness the power that is already there and then go for more power.