Bottom line:

GSM900: Normally first licensed spectrum block in Europe, Asia, Africa and Middle East

GSM1800: Secondary block in above regions. Primary block in Brazil. Also used as additional spectrum by GSM900 networks.

GSM 1900: Was Primary spectrum block for the America's (except Brazil and others like Suriname, Venezuala, French Caribbean that adopted GSM900 earlier)

GSM 850: Secondary spectrum block for the America's. Mainly defined to allow TDMA networks to overlay or replace their licensed spectrum with a GSM overlay. (such GSM850 overlay on Rural US operators also take a similar CDMA overlay to accept all roaming traffic).

GSM 450: Was spec'd to replace NMT/Nordic Mobile Telephone, which is spec'd to go greater distances. Mainly for rural scandinavian areas. So far still a work in progress, no new info.

WCDMA: In it's current form is 2100MHz. 3G networks can run in any spectrum block. Example ATTWS is building test coverage in certain US cities via an agreement with NTT DoCoMo for UMTS1900