50mm f/1.8 - less than $100 (often less than $80) and MUCH faster than your zoom. Put the strobe away.
28mm -- yeah, you'll see a difference. the prime will be faster, lighter, less distorted.
Depends on what you want to shoot, of course -- some people wouldn't be without a 400mm tele, while personally I NEED my 12-24 monster wide.