Here's what the slams would have gone as, in theory,with some projections thrown in:
2003
AO-Agassi
French-Ferrero
Wimbledon-Roddick (I think he'd have beaten Phillopousis)
USO-Roddick
2004
AO-Safin (though Ferrero, Hewitt, and Nalbandian are also options, since they lost to Federer)
French-Gaudio
Wimbledon-Roddick (tho Karlovic could have made it through-he lost to Fed in Rof16)
US-Hewitt or Agassi (that probably would have been the final without Federer)
2005
AO-Safin
French-Puerta
Wimbledon-Roddick or Hewitt (that would have been the final)
USO-Agassi or Hewitt (again that would've been the final)
2006
AO-Baghdatis (or even Kiefer)
French-Ljubicic or Nalbandian
Wimbledon-Baghdatis or Ancic (he lost to Fed in the QF)
USO-Roddick
2007
AO-Gonzalez (or Roddick;lost to Fed SF)
French-Davydenko or Djokovic
Wimbledon-Gasquet or Djokovic
USO-Djokovic (or Roddick;lost to Fed QF)
2008
AO-Djokovic
French-Monfils vs. Djokovic
Wimby-Safin or Murray
USO-Djokovic vs. Murray
2009
AO-Verdasco vs Roddick
So judging by this, Roddick would probably be looked at as a top 5 player of all time. Thankfully, we have Fed and Nadal.