In Canada, the HSDPA of the -3/4 would still be able to pick up a signal from a carrier, depending on which carrier you're roaming onto (or if you're using a prepaid SIM while there). As far as other countries which do not use UMTS 850/1900, you'd only be able to use GSM for voice and data, limiting you to EDGE speeds.
The only country outside of the US and Canada where the -3/4 would work on HSDPA is Australia, if I'm not mistaken.