being professional doesn't depends how many lenses you own or have.
I have more than 5 lenses and I have jobs booked throughout the year.
most professionals I know and are the good ones around actually have more than 4-5 lenses
its pretty shallow to judge whether the person is a lens collector/pro or just pro by just how many lenses he/she own... (4-5 lenses = most professional) then (6-7 lenses = minority professional?)
So coming back to the main topic
both the D90 and 40D are very different camera except for they are both cropped sensor.
you shouldn't just read up on the specs and features...but go and try and test it out
look into future upgrading plans for yourself too. look at the various lenses which you wish you would wanna get and the pricing. All the best to you getting that DSLR
just a foot-of-thought to share with everyone before i leave this thread.
(*Gong) if you wanna be a sifu...dun be a gearhead lens collector...and if you wanna be a professional...dun own more than 5 lenses...if you just wanna act pro...simply just scrutinize over anything and everything in CS.