The DGL-4500 supports "dual-band wireless-n" although how useful that is remains to be seen as it does not offer simultaneous dual-band operation, so you must choose between one band or the other.What is the difference between 4500 and 655? It seems like the only major difference is the 4500 does dual-band whereas the 655 does single band.
The DGL-4500 offers "GameFuel", game-specific optimizations over-top of the underlying QoS engine. In addition to that, the DGL-4500 sports a nice OLED display and future-support for custom widgets. The DIR-655, on the other hand, has been the reigning consumer-grade performance champ for a while now. The DGL-4500, in testing thus far, seems to be just as good as the DIR-655 as far as throughput.
In the end - I think you'd be best served by a build-your-own solution and use your existing router simply as a wireless AP. If that's not possible or you just don't want to deal with it, either the DIR-655 or DGL-4500 will serve you well. It's just a matter of whether you want the additional spectrum, GameFuel and OLED screen of the DGL-4500.