Well, since synchronized playback on the network was something I have also been meaning to investigate, I did a bit of googling and came up with the following link. Guess its a bigger problem than one might think. These guys have been working on this for awhile (3-4 years) and the comments here are well worth a read if you want to educate yourself on this subject. In the end, its almost an impossible problem to solve with today's technology. You might get close to acceptable synch if you have a small home gigabit ethernet setup. Wireless synch, on the other hand, is not possible.

http://snarfed.org/space/synchronizing+mp3+playback

Try AMNotebook. http://www.aignes.com/notebook.htm The Lite version is available at no cost for personal use. Commercail use requires a fee, which upgrades you to the Pro version. See http://www.aignes.com/notehelp/topics/differences.htm

I spent a lot of time looking for this type of program over the course of several years before I found AM-Notebook. I usually won't pay a fee for utility type software like this because there are a lot of decent freeware and open source programs available. But after using AM-Notebook for about a month I broke down and paid the fee for a pro version. Best money I ever spent on a piece of software.

This has everything I want and most of what you've said you're looking for, and it can be installed on an external USB device, in case you move around a lot from PC to PC. The Pro version can also use network storage. It runs continuously and minimizes to the system tray, where it's easily accessible. It has a small memory foot print, and is very responsive even on low power PCs. The author want's to keep it clean, simple to use and fast.

I've been using AM-Notebook for about 3 years now and wouldn't trade it for anything. Download it and play around with it for awhile. I think you'll find that its a very versatile tool.