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
Another option is the Ortofon RS 309D. I have heard the Jelco, Ortofon and EMT all on the same table with the same cartridge. I would rank the Ortofon better than the Jelco and the EMT better than the Ortofon
Thanks again. What improvements did you experience?
I imagine improved tonal color and bass definition with an increase in dimensionality...
@noromance not sure which cartridge do you have, but the Schick "12 tonearm was made especially for Ortoron SPU! For your Garrard 401 it would be a perfect match with low compliance cartridges, but for normal cartridges (5-10g) you need lighter counterweight. The Schick is the most ellegant tonearm on the market today imho. Here is a picture of mine.

The only problem with Schick is that you have to wait 4-5 month in line for tonearm when you buy new from Schick. 
Chakster, nice set-up. Thanks for tge info. What improvements would there be over the Jelco with my Decca SG-P?

If you feel like going hard-core Decca/London, the two arms known as great matches with the cartridge is the Fidelity Research 66 and the Zeta. There are not a lot of them around for sale, so you'll need to be patient.

One new arm popular with the cartridge is the Helius Omega. Kind of expensive at around $3200 for the Standard (copper wire, tungsten bearings) and $5200 for the Silver Ruby (silver wire, ruby bearings). A very sophisticated arm designed and made in England by a genuine rocket scientist!