I used to ride a fixed around the town I lived in. I sold it before I moved and will buy another one once I get some money saved. I dont really see the point of upgrading unless you are changing the ratio. I also ride a road bike and have most of what makes them cost $$$ is components and fixed/single really dont need any new components. A nice deep v wheelset is always nice but you already have a decent set on there.
Main thing, other than ratios, you will notice a difference with are bars, saddle, and pedals.