I upgraded the arm only on my Rega P5 from an RB700 to an RB880 keeping the same van den Hut MC One Special cartridge and everything else the same. The difference was immediately noticeable and incredible. Much more than when I upgraded the table to the one the RB880 comes standard on, the P8. I don't spend nearly the time or $$ of the other contributors here, but I will say I only play records on my stereo and enjoy improvements to the sound and try to make the most logical decisions. Tonearms in my mind are at the very beginning of the signal path, probably only less important than the cartridge (assuming you are considering the tonearm wire as part of the tonearm).
For example - no connectors within the wiring from the cartridge to the input of the phono stage, which means no removable head shells. If you are that desiring of experimentation, get a multi-arm table. If the power is sufficient, use an integrated amp instead of separating out the preamp - once again eliminating a connection point. No adjusting VTA which is another point of movement within the arm which should be as rigid as possible. Are your really going to adjust based on the thickness of every record you play? Roy Gandy says not to worry about it because of the small effect the VTA adjustment has relative to the length of the tonearm. Set it up once and forget it.
If you are more of an equipment hobbyist, I can appreciate that, but I believe some of you have gone way past the point of obsession.