25MB/s (200Mbps, so looks like you're already above 100Mbit) is actually pretty good for a single network attached hard drive for copying a bunch of files up to it, you aren't going to get a whole lot more without getting into a more expensive NAS with multiple drives. The theoretical maximum of the drive is 100MB/s.
The speed will vary quite a bit depending on what you are doing.
For syncing the My Music folder, is there a good reason to sync it at all (i.e. is your PC a laptop)? Otherwise probably easier to just keep your music on the network drive. If you do want to sync, I think Windows can actually do that (Offline Files), where Windows will make a local copy of a network share and automatically connect or use the local copy depending on if you are connected to the network or not. I've never used it though.
The Cisco router/modem you have already has Gigabit ports.
EDIT: Too slow
