What does a tonearm contribute to the sound of a turntable?

Turntable supports the record, the tonearm serves the cartridge.Some cartridge makers even suggest using a particular type of tonearm - sometimes due to mass or pivot type.High end MC cartridges will need an excellent arm to deliver all of what they're capable of. I recommend buying the best cartridge you can afford and buying an arm and table that is suitable to deliver the goods.Most people want to spend the money on a table because $10,000 tables are more fun to look at and interact with than $10,000 cartridges.The cartridge is the transducer turning mechanical energy into electrical energy. The arm is well, an appendage. It needs to be good enough to let the cartridge do its thing!

@2channel8 The Jelco 850 and JAC501 cable will do what you are looking for. It is a very good arm. George Merrill has the best price. 

I agree with Simio and FsonicSmith above - I had a VPI Traveller turntable for a while and it was the 2nd Worse TT I've owned out of dozens. The worst was Nottingham Horizon which had a motor that caused vibrations on the tonearm so bad you could feel it.
After just a few days with the VPI Traveller, I lost a channel. I first suspected a cartridge lead - those were all okay. Then I checked the phono preamp tubes - all good there. I finally figured out that the phono box in the back of the Traveller had a loose connection.
I removed the two wood screws holding in the panel and I was so disappointed to see such a poor and flimsy soldering job. And this turntable is called the Traveller? The phono leads on my Rega RP10 are integrated and it sounds great to me. I don't get the point of removable cables on budget decks anyway. Besides, who's going to get a better deal on high quality cable, say from Cardas, a factory who buys in bulk or Vinyl Joe?

If the Jelco dropped the noise floor, you should most likely have to turn the volume down, not up.