thank you all again for taking some time to respond. I have been doing a bit more research and i think i have narrowed my choices down to 2:
Because my project is based on the BOE-BOT the easiest choice would appear to be: the digital encoder kit made by parallax http://www.parallax.com/Store/Robots/RoboticAccessories/tabid/145/ProductID/80/List/0/Default.aspx?SortField=ProductName,ProductName
I have some reservations though that it will not be the most accurate system. But I do believe it will be the most economical when considering time and other factors.
The other way I am thinking of going is with new gearmotors such as these: Pololu - 75:1 Metal Gearmotor 25Dx66L mm LP 6V with 48 CPR Encoder (No End Cap)
They are slightly longer then my current servos so I think they will require some amount of modification to the BOE bot chassis, in addition if I am understanding correctly it seems i could not run them off the regulated 5v provided by my boe bot arduino shield because it provides up to 1A and each gearmotor is listed as using 40ma running and 2.2A stall. Am I understanding correctly that I need to provide for 2.2 X 2 or 4.4A regulated 6V? (although the motors would run ok on 5V) Also with regards to the motor controller I would think I would need to have one to control the motor RPM?
Is it likely accurate then that to go the gearmotor route I would be looking at getting the motors, doing chassis modification (or replacement), would need some form of DC motor control board and possibly another batter pack depending on consumption and the input range on what ever is regulating the voltage to the gearmotors?
I am leaning towards the parallax encoders and am curious if anyone here has tried them out and has any feedback about their error rate. I am thinking i will start there and for the next project plan ahead for the gearmotor / encoder route.
Thanks again,
Brian