Celeron vs. Pentium?

How does a Celeron CPU compare to a Pentium? More specifically, unless you’re doing any serious gaming or number crunching, does it really make a difference? I ask because I am planning on getting my dad a computer and I think that a Celeron might be plenty for email, wordprocessing, picture editing and basic spreadsheet type work. Do you guys think I should spend the extra $150 and get the Pentium? (By the way, if I could purchase a Mac for under a $1000, I would do that).