For under $500, you can basically categorize as follows:
1) External HDD devices like the WD / Lacie etc. These are basically just an external hard drive with a network connection. They work fine, though don't offer functionality like redundancy. Ideally you'd buy two, or keep a copy of all files on your computer too in case it dies.
2) ReadyNAS / QNap / Other Linux RAID NAS devices. These are fullscale mini-servers with print servers, web servers, RAID5/6 redundancy. Capable of running Slimserver should you ever swap to Logitech/Slim/Squeeze.
3) Windows Home Server devices like HP Media Smart, Tranquil, Acer. Essentially the Windows version of (2) running Windows Server 2003 with Home Server additions. Redundancy is available through automatic file duplication to the pool of drives you plug in. Install the Connector software on all your Windows PCs and they will be backed up each night (restore is boot CD, select backup from server. Pretty nice). Can run Slimserver should you switch, and has a host of add-ins for auto-backup to Amazon/Jungle and of cause RipNAS. RipNAS looks nice, but if you already have paid for dBpoweramp and prefer to sit at a desktop/laptop to rip anyway doesn't seem to add much value.
Myself, I've been running the HP Media Smart WHS for 20 months and it has been great. Hot add/swap of drives is nice, though not necessary, and it's a very nice case

The fan noise is loud on mine (and not on others, it seems to be a crap shoot) so it sits in another office. I use the JungleDisk addin to offsite backup our photos and docs to Amazon every night, and as well as using file duplication for all files over 2 drives I also use the builtin server backup to backup to a USB drive to store offsite at work. This gives me local redundancy for music on the second drive should the first fail, and also a full offsite backup. Since rips can be recreated, but photos not, the additional backup at Amazon gives peace of mind.
If you are happy using dBpoweramp then any of these would be a solution for you. Though for the price of drives these days, I would always advise on keeping a backup of everything. The WD Green drives are nice and quiet, and reliable, and although just 5400rpm are more than fast enough for serving our 4 Sonos zones simultaneously.
Cheers,
David.