Couple things to add...
Most common radios in use on current sprint phones...
1x/EVDO at 850 cellular frequencies... supports roaming on verizon towers (ongoing support thru at least 2016)
1x/CDMA at 1900 PCS frequencies... primary sprint tower frequencies, ongoing
Wimax at 2500 frequencies... only present in 4g wimax phones (support thru 2015)
1st round of network vision
-1x-advanced will be added in the 800 mhz SMR spectrum (they couldn't deploy LTE here because LTE requires a minimum of 10mhz). 1x advanced gives sprint more connections per tower and provides support for HD voice (the network and the phone must support it... a la the Evo LTE).
-LTE will be rolled out in a 10mhz (5x5) block of the 1900 G-block spectrum AKA band 26. They have pretty much nationwide coverage in this spectrum (which IMO means it is and will continue to be the primary frequency for LTE long-term for sprint). Coverage/penetration should be similar to todays 1x/evdo but should have a bigger foorprint.
Network vision, later on...
-The next round of rollouts is expected to be LTE in a 10 Mhz (5x5) block of the SMR band. This will happen as frequencies are freed up as a result of moving Push-to-talk customers onto CDMA and off of iden. No phones today will support this because it requires channel aggregation technology that's not yet available.
Here's a link to an article about this 2nd round stuff... this is a ways out and there are a large number of dependencies before this work starts:
Sprint will activate LTE on the 800MHz band in 2014 - Engadget
A lot of people really seem to be worried about the 800mhz LTE but it seems like an odd thing to be worried about IMO. 1900 LTE is where it's at. 800 LTE will just be a little icing on the cake... of course that's just my opinion.