I was going to purchase the 7300GT for my new computer, but I noticed today that the 8500GT is just $20 more. I'm not a huge gamer but I'd pay the $20 if it was not only worth it but if the card would last me longer.
It really depends what you do with it. If you don't play games, I don't see why it'd be worth it "just to have DX10". And if you do play games, they're both very weak anyway.