What tonearm will better mine?


I have been running a 12" Jelco SA750L with an upgraded Decca Super Gold on a Garrard 401 for a few years. I'm happy with the sound and happy with my pre, power and speakers. (Croft 25R, EICO HF35s, -good tubes, Quad 57).
I keep hearing that my tonearm is a weak link even though it sounds pretty awesome to me. What would it take to replace the Jelco and make a significant improvement? What should I expect to hear better? Would I have to drill the massive plinth? Thanks.
128x128noromance
Thanks all. No one has actually explained what sort of specific improvements one should expect from a new (expensive) arm. I think I'll order the low-slung counterweight and carbon fiber headshell for the Jelco for the moment and spend the difference on more records. 
Expressimo Audio makes a copper end stub and counter weight for your Jelco I purchased mine last year it made a major improvement. The arm is much more stable.
Thanks cjaronica. It looks like it means dismantling the arm off the table. Interesting though.
Why not first get a very good tonearm cable? the internal wire is decent stuff (silver I think) but a nordost tonearm cable will be a big jump forward, as would a furutech for instance