1.) The exact same sticks. Only try to mix and match if you can't find them anymore. If that's the case, grab a set with the same specs (voltage, timings, speed, everything).
2.) If you're adding another GPU, probably. The OCZ aren't exactly the best quality, so they might not handle it. To make sure, find the link to the PSU wattage calculator in the stickied thread called "Guide to choosing parts". It's a very accurate and detailed calculator.
3.) Personally, I don't think it's worth it. Chances are you'll want to get a card from the new generation in a year or so, thus defeating the purpose of adding the second card. Besides, you likely won't be able to sell it later. Or at least not for very much.