are you running any other lights? Anything for atinics, etc...?
Or as chad said, what color do you want? Yellow, white, blue, purple?
Also, ask around on the ebay bulbs. If I'm not mistaken, most have MUCH less time on them than the reputable names.
If you're looking for a blueish/white look. (not purple, not yellow) might want to look in the 14k range