The term "520 conversion" means a couple of different things to different people. I hope to clarify a few things. First, a 520 conversion doesn't necessarily designate a change in gearing. For most people, the "width" of the chain/sprockets is what we'd refer to as 520, 525, or 530, etc. to make things simple (the technical term is "pitch"). The idea here is less weight on the drive train which is suppose to translate into better performance. Generally 520 sprockets are aluminum (which means they wear down faster) over the stock steel sprockets but they are lighter. A 520 conversion is generally a pretty performance oriented modification for your motorcycle so it really depends on your riding style and what you want to achieve with modifying your drive train before choosing the right set up. Remember, all the components of a chain kit (front sprocket, rear sprocket and chain) need to have the same pitch to work together.
A very popular tweak to the 520 conversion is to chain the gearing of your sprockets or the number of teeth which works miracles on acceleration of middle weight 600/750cc sportbikes. The consensus is that -1/+2, down one tooth from stock on your front sprocket and up two teeth from stock on your rear sprockets, is a good place to begin for most 600's. This is a great set-up for the performance oriented riders as it adds a big bottom end punch (helpful for these newer top end friendly bikes with little bottom end) and will bring you through the rpms/gears very quickly. This adds to the already frenzied 600 attitude but I absolutely love the way it feels. It's not a crime to run -1/+0 if you're just looking for a little boost when commuting or riding with your buddies. -1/+1 is also a popular choice.
I found that some of our customers on the big bikes (1000cc) like to do -1 in front OR +2/+3 in the rear as it lets you gear up, or use a higher gear than you usually would, while riding. This is a great benefit as most of the big bikes do about 100mph in first gear and screaming along at 11,500rpm with a hair trigger throttle and 180hp can be a bit unnerving all while at full lean. The low end is improved for your daily riding as well.
I receive a lot of questions about which brand of sprocket or chain to use. As far as the sprockets go, any hard anodized sprocket will do. Some of our top sellers are Driven, AFAM and Vortex for aluminum 520 sprockets. Front sprockets are generally steel. With chains, like most things in life, you generally get what you pay for so don't skimp on the chain and it will serve you well.
SoloMotoParts.com offers 520 conversion kits that allow you to pick your sprocket and chains in various colors and teeth size to serve your needs. See them for Yamaha, Honda, Suzuki and Kawasaki.
What do I run? I use Driven/AFAM sprockets (-1/+2) with a DID ERV3 chain on my 06 R6 track bike. Also, note that changing your gearing will make your speedometer read wrong. A Speedohealer will fix this.