The aqdvantages of upgrading to a 2xxx are many from the 740.
1)harmonizer. Even if you don't sing, this input can be used for guitars or line-inputs from other instruments
2)interface. Larger screen...more direct access to most real time features
3)better speakers
4)more memory for styles
5)new sounds
.....this list is subjective, but goes on from there.

The Tyros is another step forward (maybe a leap)adding:
1)hard drive
2)double polyphony
3)new mega voices
4)better mic input
5)better key feel(I'm told)

The 2100 will cost you abought 1/2 of the Tyros price. You'll have to decide if those extras are worth it to you.