In the case of Cingular in the USA:

64K SIM: The only benefit to a 64K SIM is ENS, which allows Cingular to
manually "home" the phone OTA (over the air) to either the "blue" (old ATTWS
network) or the "orange" (old Cingular) network, but only with newer
ENS-capable mobiles.  (Even without ENS mobiles will still roam freely on both
networks, but not necessarily as well.)  Except for that, a 32K SIM works just
as well as a 64K SIM, and when the two networks are finally merged (as
expected later this year), ENS will become pointless, eliminating the benefit
of the 64K SIM even for ENS-capable mobiles.  There's no additional benefit to
a more costly 128K SIM (since Cingular isn't using SIM-based applications).

3G SIM: Needed for the new Cingular UMTS mobiles.

My point that I was trying to get across, was that T-mobile GSM Phones in
the US are the same phones they use in Europe and Asia.  I coined the word
"generic", because they work almost anywhere, except Japan.  Compared to
Cingular/AT&T GSM phones of the same model family that don't always work
outside of the US.  T-Mobile tends to use Software rather than Hardware to
limit it's phones, which makes them more universal than Cingular phones.

For example, I purchased a couple of Cingular 3620 and 3650 phones.  They
did not work outside of the US, and after doing a conversion, they had other
problems.  I learned my lesson and no long buy Cingular Phones for export.
I buy mostly T-Mobile phones for export and up to now have no had any
problems unlocking and using them in GSM countries.  My last purchase of
Nokia 6600s were all T-Mobile and made in Finland, most of the Cingular
Nokia phones are made in China or Thailand.