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The Sylvania Xtravision series use the highest wattage (55/65) filaments that are legal. They will give you better illumination under normal conditions. (Foggy conditions will require yellow or amber fog lights.) They will put even more strain on the stock wiring, so relays are a must.
You can easily find higher power bulbs that draw 80 or 100 watts, but there is a good chance that they will melt your plastic housing.
Whatever you choose, you should avoid those bulbs that have been dipped in blue paint to alter their colors; they are not legal, and provide very poor illumination.
Don't buy from these guys. One of my Yellow SuperBright ION bulbs burned out in just over a year and they wouldn't replace it because the warranty ran out. I ended up putting my 12 year old bulbs back in and they are still going strong. EuroLights.com - True High Performance Lighting. Here's a good source for a relay kit Catalog