well, I'm not really sure how, but Alucard can be killed. He can also be bound into service or other things. He doesn't age, as he has complete control over his form...
metamorphmagus in a completely new direction?
Either way, Alucard is just as immortal as the Highlander, and only slightly more so than Volde.
Blast up Alucard, and he regenerates himself a few seconds later.
Blast up Volde, and his followers regenerate him a few years later.
with the horcruxes, Volde shouldn't be aging either, and we know that Alucard doesn't age.
Destroy Alucard on Blessed grounds with Blessed silver weapons and use seals to prevent him from regenerating, seal his power and launch the remains into the sun? I think that will kill him.
Destroy all Volde's horcruxes, then kill Volde. viola, dead volde.
When Alucard says that all things die, he means that anything that doesn't die on its own get killed by something else.