If you want to index 10 speeds, you'll need to buy a 10-speed shifter, 10-speed cassette, and 10-speed chain. That should be it, if you have an 8 or 9 speed system now.
(The Shimano 10 speed bar-ends are only $50 right now at Nashbar, I just converted my and my girlfriend's bikes to 10 speeds.)

A 10-speed road shifter will perfectly index 10 cogs with your existing Deore RD.

So, changing the RD will have no impact on your ability to index 10 speeds. If your RD needs to be replaced, replace it. Changing to 10-speeds isn't a reason to upgrade it.


Shifters control the indexing (number of speeds).
* Shimano SIS is cross-compatible: 7, 8, 9, 10 speed Road, and 7, 8, and 9 speed MTB.
* Shimano Dyna-Sys is 10 speed MTB, not compatible with SIS.

I'm not sure what they call 11 speed Road and 11 speed MTB, but they're not compatible with anything else (AFAIK).