Edited by eprex - 4/16/13 at 11:04am
I say keep your dress shirts and wear them casually only. If the sleeve length bothers you a lot, get them altered. I am also 5'9" and 150 lbs (40 chest, 32 waist) so I understand that it's difficult to find the right size.
For future shirts, I would recommend either going MTM (moderntailor, tailorstore, mytailor) or getting shirts that fit as best as possible and getting them altered. I like going the second route. You can get shirts anywhere from $30 (Nordstrom Rack) to $70 (Brooks Brothers on sale, Charles Tyrwhitt, TM Lewin) and then pay for additional tailoring. Darts should be about $10-$15 and taking in the sides should be $20 or so. I usually have to take in the sides and get darts which makes the tailoring job for each shirt $30.
You went about it completely backward. Find a shirt that fits in the shoulders and chest and have it darted by a tailor to fit your midsection. These shirts will never fit you. Buy new ones.