Chromatics are best man. What a simple scale but yet at high speed can sound better than any other. Petrucci uses ´em frequently.

Also do you know that 4 string sweeping part from Beyond This Life solo. Would you call that atonal. I´m sure I would.

And that crazy speedy melody on Beyond This Life after (I think first chorus). That one is really wierd and death hard to play. Sounds VERY atonal to me. Even Petrucci said on Scenes From A Memory DVD that that melody is one of the hardest and most wicked guitar part of the record and show for that matter.