I think he could make a good Voldemort, I've seen him in a number of films; Dangerous Liasions, Hart of Darkness, Mary Reilly, The Portrait of a Lady and Joan of Arc The Messenger. All of which are films based on or adapted from classic novels and historic literature so I definitely think he can do good portrayals or written characters and the more I imagine it the more I can see him acting as the older, somewhat decayig, bitter, frail and plotting man Voldemort is. He's shown already (e.g. Mary Reilly based on Dr Jekyll and Hyde) that he can play a man who wants youth and release from torment, who has a desire and ambition to be more then he is and reclaim a past so I think he can do it again. 🙂
Although, of course he'll have to be even better this time (admittedly he was great before but still lol) just because it's Harry Potter and we have outrageously high and sometimes unfair expectations of it! 😛