I would suggest purchasing from DDM Tuning (
http://www.ddmtuning.com ), as they have a lifetime warranty on their products and they're a fairly established company. The majority of us in the Altima Coupe section use them and haven't had any problems thus far. They're prices are pretty competitive too! I just hate to see people spending money on these fly-by-night eBay sellers that may or may not be distributing a quality product...
With regards to what 2young2retire said regarding the bulb output "colour", he's mostly correct. The light from HIDs is noticeably blue around 8000k. For a nice, crisp white without any hint of yellow I'd suggest 6000k. Yes, light output is indeed less the higher you go up in K-value, but even at 12,000K (close to purple) it'll still be brighter than the halogens currently being used! For the purposes of general driving, the DDM 35-watt kit will be perfect.