And you should also be able to polish the outer lenses with a good cutting compound, if they are showing signs of 'yellowing'. This question has been asked a number of times, so I would also suggest you use the search fuction for more tips on how to do this properly.
genuine Xenon headlights cost anywhere between £500 and £1000 - EACH!!!!
Give them a good clean with washing up liquid, then use T-Cut on them and see how they look. If the browning has gone you will notice a much better light. And as already posted fit the Phillips bulbs.
If you dont care about money just buy the HID kit which a few people have fitted and say its good or pay up to £1000 for a set of Nissan HIDs.
I dont have to tell you which i ended up doing! lol