I don't see how spending money on a working bike to change it into something it wasn't meant to be is going to make you more interested in riding. If you think that fixed gear will be fun enough to make you want to ride more, you're probably better of scoring a fixed gear off of craigslist and keeping your old bike as well. If you don't like it, sell it. If you're smart enough you can probably come out even or ahead. If you like it, fix up your old bike and sell it, or keep it and have one in case the other breaks or you need a specific bike for a specific job that it's better for.

When you can get a decent fixie on Craigslist for 250-500 bucks, why spend the time and money to change a working bike when you can have two working bikes?