Invitations Should be Written, Not Emailed!
Do you remember when people used to send out things like birthday invitations and party invites? These days, it seems with email, people skip that formality and just send somebody a quick email. I don't know, I like the formal invitation. I think it's a nice touch to still get something in the mail like that. Look at wedding invitations; you don't see those being done online as much as others. I guess it's because weddings are more formal, so people want something special.
Birthday and party invitations can be special too. It's just finding nice cards, and taking the time to write them out. I just did a large mailing project for a site I own, and handwrote about 100 envelopes. I think it's a nice touch to take the time to handwrite something out. It shows people you took out more than one moment of time for them.
If you're looking for a nice selection of invitations, try ThePartyStartsHere.com.