should I get 3:90's ??

all depends on how much highway driving you do IMO...3.90's are definitely going to allow you to get up to speed in a hurry, provided that you don't spin too much...i'm about to swap back to 3.73's...didn't like 3.55's, and the 3.27's in my newly installed rear take too damn long to wind out...
 
I got 4.10's behind my 5spd and like it well.
RPM at the end of the track will be dependant on tire size and mph.
What times are you running now?
 
If you make peak power at 6000 rpms, I think you would want to cross the line somewhere around there. Determine your gear ratio based on that. I know that not too specific, but I'm sure someone else can help you out more. There's plenty of online calculators that will help you out.