It has built in storage... some will be partitioned off for system ROM and user data apps .. then the rest probably about 14gb or so after formatting will be left for pics music and apps to sd(if you can even use the internal user storage as apps to sd)
Its listed as not having an expansion slot for added storage... thats a - from the evo and most top level smart phones... hardware wise they should have gave the phone better specs...
It does not even do HD video recording something the galaxy phones do do and are specked almost the same...
If i were you and going to t-mobile the better phone would be the my touch 4g
google has a comparison page out
Compare Phones
this compares the 3 nexus s my touch 4g and evo... the my touch 4g is better or almost as close to an EVO that you are going to get on t-mobile...