Where to store large photo library that also can be backed up?
A few thoughts come to mind.

Firstly, this is all dependent on what sort of software solution you're using to store your photos. If it's just files in folders, that makes things simpler, and is pretty platform agnostic. But if you're using (or want to use) some sort of organization software, then that obviously complicates things, both in terms of platforms and performance. That said, here's what I think are the best options.

PC as Network Storage

This is what I do (for videos and bulk archival, not photos, but the concept is the same). I have a generic PC with a bunch of hard drives running headless off in an unused room, connected to my network. It runs Linux, and while I have it set up to do various automated tasks, it's main purpose is to be a big network drive I can dump stuff on.

It sounds like this is more or less what you have already. If your current hardware isn't adequate, then look into upgrading. For file serving duty, most old hardware should do just fine (mine is a dual core Celeron circa 2008), the biggest bottleneck is network speed, make sure you have Gigabit and the file server software is behaving (I've found that file sharing between Linux and OS X is much faster over AFP than SMB).

Getting it quiet can take a bit more effort/expense, but it's definitely doable, particularly when you don't need a lot of horsepower (basically, get the biggest heatsink you can, and use a small number of very large fans spinning very slowly).

This option is definitely the best value for your money, but likely means more time invested on your part.

Mac Mini

Using a Mac Mini in basically the same role as above has some merit as well. It's a bit more expensive, and less expandable, but requires less time investment for set up and maintenance. And of course it's a much more compact form factor than your typical PC, and pretty quiet unless you're doing something very CPU/GPU intensive. You can get up to 2 TB internal storage in one (doing so from Apple requires the more expensive "server" model, but you can save a bunch of money by getting the base model and installing the drives yourself - it's fairly straightforward).

In addition to simply acting as network file storage, you can use iPhoto to organize all your photos, and share them over the network to your other Macs. If you add OS X Server ($20 from the App Store), you can have your Mini act as a Time Machine back-up server for your other Macs (I haven't tested this, but I've heard it's much more reliable and has better performance than using a Time Capsule).

Other Options

NAS Appliance

As you've seen there are a wide variety of these. The cheap ones don't have very good performance (few even come close to saturating gigabit ethernet) and lack expandability, and the quality ones ramp up the cost to the point where using a PC or Mac Mini seems to make more sense in most situations. The main selling point seems to be their relative set and forget nature.

Time Capsule

I've never heard very good things about this. As a network storage appliance, it's lacking in performance and expandability. It's major selling point is integration with Time Machine, but its reputation seems to be spotty at best as a back up device. Unless having one device as your Wi-Fi router and network storage really appeals to you, I'd recommend against it. Likewise an Airport with attached hard drive — if you already have one, then it might be worth trying out to see if it meets your needs, but I wouldn't go out and buy one just for this.

Backups

For backups, keep things as simple as possible. External drives are pretty cheap, buy one and plug it in to whatever stores your photos. Set up some sort of backup script or program to perform regular backups (rsync should suffice if you're just backing up your photo storage — some sort of cloning software might be better if you want the whole system backed up). For added protection, do backups to second drive every week or two and keep it at your office or somewhere else. Alternatively, look into online backups such as Backblaze, CrashPlan or Arq, which is less labour intensive, but can be more costly and/or problematic depending on your internet speeds and cap.