Just finished up a Glock 34 for that will be a competition gun for IDPA.  The customer and I decided to add a set of Dave Sevigny Competition Sights to the gun.  These sights are made by Warren Tactical and feature a plain black rear sight with a fiber optic front sight.  The sight comes with a red and a green tube for you to choose.  We settled on the red tube, and I think this suits my eyes better.

Internally we did a competition trigger job that makes this gun incredibly fast on follow-up shots.  This includes setting the overtravel for the trigger, and customizing the springs for good reset and ease of pull.  This setup is all you need to make a substantial difference in a gun that is great out of the box.

Cost: $200 for the sights and trigger job.