Without going deep I think Dexcool is better than Volvo coolant, and indeed is a much better bargain. It certainly lasts longer than Volvo coolant and has less tendency to go acid than reg. coolants.

Indeed make sure you get every last bit of the old coolant out. In fact when I change my cars over, I find it a good time to replace hoses, thermostat, I even like to put a new water pump, goose neck, and other new bits on the older beasts that are all deposited up. Be sure to purposely reverse flush the heater core, block, radiator, turbo cooling lines etc. It's not that the Dexcool is incompatible, it's just that some of the longer life benifits are negated by mixing with regular coolant.