I'm with PaulVS here...
My list would (and did) include:
1. Progressive or Race Tech front springs, c/w oil change. I used Progressive springs and 20% SAE 20 mixed with 80% SAE 10W fork oil to raise the viscosity slightly.
2. Rear Shock - I went the very low cost route and put a B12 shock in my 600, mostly for the stiffer spring, but the 'busa shock is a way better upgrade.
3. Brakes - I used EBC HH pads (more initial bite) and SS lines (more force/better feel) to update the little 600. Fresh brake fluid keeps things working better.
4 Tires - the factory fitted Bridgestone BT-86 in the little 600, which were decent. I have used BT-20/21's as well as Pilot Roads. I like the Pilot Roads best so far...
Then I went the jetting route, and I am looking at installing a slip-on this winter.