I'd consider 3 specific areas:
Phono stage - this can make a huge impact alone
Table - I recognize you have a modified RP3, I upgraded a little while ago to the P8 and the improvement in the plinth and tone arm over the P6 was noticeable as was the upgrade from the P3 to P6 using the same catridge. I don't know the details of your current rig so you may not expeirence the impact I had when I moved from the RP1 to P8.
Cartridge - Can be a huge impact; and if shifting from MM to MC you may want to split the $4 between phono stage and cartridge.