Yes, you can use a 9 speed crank with an 8 speed drivetrain. A lot of people do it and it works fine. Some people report that if they are in the middle chainring and try to use the smaller cogs on the cassette, the wider 8 speed chain will rub against the large chainring. This will depend on the chainline of the crank, it would happen more with non-external BB cranks with chainline of 47.5 mm compared to new external BB cranks with 49-50 mm chainlines. If rubbing does occur, it is easily resolved by using a 9 speed chain. 9 speed chains will shift fine on 8 speed cassettes so no worries there. If you do use a 9 speed chain, your front derailleur may shift better if it was replaced with a 9 speed FD too as it will now be shifting a 9 speed chain on a 9 speed crank. I am sure you would be able to get the 8 speed FD to work good enough though.