The Park Tool link that neil0502 provided is a good one. Spend some time adjusting the derailleur properly - start right from the beginning, detaching the cable and all. Sora will shift very nicely, especially with a Tiagra RD (the Sora RD is the only truly crappy part of the group).
I'm all for getting new stuff, but in this case I don't think you need it. Poorly adjusted Ultegra will shift just as badly as poorly adjusted Sora.
