Mostly you need to decide which features are of most value to you. There is little difference in the major brands. Failure rates and performance are pretty much the same. If cost is not your major concern I would go with gig ports and dual band just for future compatibility.
The short answer is you can run a mix of gig and non gig devices and it will all work.
The longer answer depends on a lot of things. Obviously if you have one device at 100m and another at 1g you will be limited to 100m between these devices. Not by the router but by the 100m device. If both devices are 1g you can to a point use 1g. There are technical limitation that will prevent you from getting to 1g without changing things like the MTU but it is unlikely any home machine can even come close to 1g. This again is mostly the end clients limiting it not the switch.
The gig ports on the wan side though are mostly for show. First nobody will be able to afford internet that can really run over 100m. Second these small routers can easily SWITCH at 1g but they cannot route, especially if they are running NAT on the packets.
Can someone recommend a replacement for my DIR-655 router?
A few years ago, one of our more prolific moderators recommended I purchase a D-Link DIR-655 wireless router. Until now, it has provided uninterrupted, flawless service and I've never regretted getting it. However, the wi-fi component seems to be dying. I've run several tests, but I think the wi-fi component is really taking its last bow.
Now I'd like to replace it with another D-Link router, if possible, but I am willing to consider other brands. Like the DIR-655, I'd like the replacement to support 4 (or more) wired devices, have Wi-Fi, and Gigabit Ethernet speed even if some of the other network devices are not Gigabit-capable. I mention that last requirement because I've been led to believe that many Gigabit routers will support that speed only if all the devices in the network are Gigabit-capable. However, some of my devices are not Gigabit-capable.
Any suggestions, anyone? Thanks in advance.