Bottom line is, a narrow band sensor will not give you accurate readings. Basically...buying an autometer gauge or hooking up a volt meter is just a waste of time and money and will lead you in the wrong dirrection. The only true way to monitor a/f is by EGT or with a wideband a/f sensor and kit.
I purchased my kit from PLX and it was worth every penny. I got the extra wireless data logging feature and spent about $500. However they do sell basic kits with a display for around $320. IMO thats a steal....
But any kit that uses your stock sensor or taps into the stock signal is worthless. A good kit should include a wideband sensor, and a brain. The sensor is a 5 wire sensor, and range in prices from $80-$200. The majority of the money spend is on the brain which takes the signal and makes it into something readable and understandable.
Good luck on your choices man, but I'd spend your money on a useful gauge such as oil pressure or water temp and ditch that gauge you just bought. Its going to blink like crazy, flash, run up and down, and just be an eye-sore. I'd sell it to some ricer.....but everything is just my two cents......