Senators who won "squeaker" races in 1998
Boxer (CA)
Reid (NV)
Lincoln (AR)
Edwards (NC)
Feingold (WI)
Senators who might retire in 2004
Hollings (SC)
Daschle (SD)
Inouye (HI)
Leahy (VT)
Miller (GA)
The Republican side is in much better shape, not just for the lower number of seats. Of the incumbents should they run again, I would only put Bond (MO), Bunning (KY), Fitzgerald (IL), and the Alaska seat into any kind of play. And given the newly won majority, I don't foresee many on the GOP side deciding to retire.
I would take out Bunning. There was an article a few weeks ago saying the democrats have no candidate to run against him. However I would add McCain (Az) and Campbell (CO) due to impending retirements. McCain's health is deteriorating and his presidential ambitions are shot. Campbell is reported to not be raising any money, something senators must constantly do to run effective campaigns.
Republican also have a shot at knocking off Schumer if Guiliani decides to run. Your also forgetting Bob Graham of Florida who is getting much older and will likely retire. Dittos for Barbara Mikulski, though Maryland is a tough state for republicans.