Q: I've been invited to a Indian wedding. What should I wear?
A: Wear what you would wear to a non-Indian wedding. The only clothing taboo to be aware of is that you must take your shoes off to enter a temple or the wedding canopy, so be prepared to leave them at the door. Do try to make some allowances for comfort -- it's not uncommon for seating to be on the ground, so a tight suit or dress that doesn't let you sit cross-legged might not be the best choice.
Of course, if you're the adventurous type and want to wear traditional Indian clothes, go for it! Very likely there'll be a friend or cousin your size who'd be delighted to loan you some clothes and show you how to wear them. Women guests may even get the chance for an application of henna or mehndi if they're around the day before the wedding. But don't rely on the bride or groom to help you themselves -- they'll be way too busy on the day of the event.
Q: My Indian friends are getting married. What sort of gift should I give?
A: The traditional gift at an Indian wedding is money. The amount should be an "auspicious" number ending in 1 -- $11, $21, $51, $101, $201, $501, etc. (or similar numbers in rupees, pounds, or what have you). Cash or a check is fine.
However, outside of India most Indian couples understand that giving money can be considered tacky and they don't expect it from their non-Indian friends. They've embraced the western tradition of giving housewares as gifts and many couples register at department stores. So the better answer is: give what you would give at a non-Hindu wedding.