Very strange. Sounds frustrating. Since it is a boot problem, I'd normally suspect a theme (did you install any?) as some older ones replace the bootx file. Still, that would cause a hang at boot _every_ time. Guesses:
1. Bad ram - is this a recent problem, or did it occur since you installed the ram?
2. Bad prefs - try creating another user and logging in as them - does the problem still occur? If not, stick with the new user ID, re-register all your apps and delete the old user account.
3. Keep the console open and see what it has to say. Did you check your crash logs (I forget where they are kept - maybe someone else can tell us)?
4. Do you have any new peripherals (USB, Firewire,etc.) or non-apple keyboard or mouse? Any hardware mods to your system?
Hope this helps.
