This is a tough one because the old etiquette rules are not to bring up gifts at all in such an invitation - if people wish to bring one they may but they should not be so crass as to think you are having a party to solicit gifts.
Some etiquette experts think saying "no gifts" is inappropriate because it brings up the subject of gifts, and inadvertantly makes people think, "Oh, they are saying that but of course we need to bring one".
For my son's graduation party last weekend, I did not bring up gifts. About half the people we invited came, the other half did not. Now I wonder if gifts were the issue, mindful of the economy. I didn't think of the gift issue either, until after the invites were mailed - never occurred to me to even think about gifts. We just wanted to have some fun to mark the occassion. Most of those who came brought a card with some money for him.
So now, I'm thinking perhaps had we said "no gifts" more of the people we invited might have come.
I think it's a poor decision not to go to a party for that reason though - among those that did not come, I'd invited entire families of four and more people - and given that I provided dinner, even if they'd stuck ten bucks in a card, it would have been a bargain to feed their families, have drinks and entertainment, and a family outing with friends for that!