The KHS Flite 800 is a pretty good bike already. Like you have already decided I would experiment with tires. I ride Hutchinson Sector 700x28c tubeless and Challenge 700x27c Parigi Roubaix, they also have a Challenge 700x30c Eroica. The Challenge tires have changed names. Lots of people like the Continental tires. When your happy with tires the next logical upgrade would be wheels.