It all depends on the investment...
If you have everything in ALAC (lossless) then you should be just fine with the AppleTV and shouldn't really be losing much in the way of quality since your Receiver will be acting as the DAC then (assuming the digital output on the ATV is used). You would, however need to set up your USB hard drive as a NAS drive - for which there are many options... the easiest of which is to simply get a USB-to-LAN server/converter... some are cheap, others are a little more... none of them are more than $100 however. If you can't find a decent option doing it that way, you can always just purchase an external drive box that is SATA-to-LAN designated and swap the drive into it. Something like this:
Network Drive Enclosure
Unfortunately, the USB on the ATV is now only for upgrades and cannot be used for connecting additional storage.
However, there are several other options available if you're willing to spend... and a few even if you're positive you want to keep it all on the cheap. If you just wanted to be able to stream from your MacBook while using it in another room, or on the other side of the room... you could just pick up one of these and connect it near your receiver and use the optical in from it.
1) Build a very cheap PC to serve iTunes to your receiver as a dedicated system... you can even find some off the shelf options. Basically just look for something that is fanless, and preferrably 1.8GHz or faster (recent Atom 525s or AMD has one too - forget what the cpu is called however). Then just hook the USB drive into it and let it sit. Just make sure it has digital output (most do at least have optical, but many have HDMI and coax as well). Something
like this would work well - and you could even use it to play HD video to your TV as well if you wanted to.
2) Get one of the many $99-199 devices that's not an Apple product, but that does have playback of either ALAC or another lossless codec. If you need to convert files, there are utilities, and providing that you are willing to wait - should be able to convert any lossless file into another one. Not sure for OSX but I know for windows you can convert anything to anything with ease using something like dbPowerAmp Converter (in batch mode I can convert ALAC->FLAC or FLAC->WMA Lossless at somewhere between 60X -130X speed - or about 25-40 seconds per album).
3) My personal choice in the near future... just save up for awhile and then drop some coin on something like the
Marantz Network Audio Player. Sure it's even $100 more than a mini... but I think it will not only sound better, but you'll have the fun of being able to just stream to it via bluetooth later on when you get that ipad...
In reality there are several other options too... just do some searches for "lossless audio streaming" or "AirPlay lossless" and you should have lots of options from $79 or so all the way up to $1200 or more. You could even just pick up
one of these and then use your MacBook wherever and use it to stream to your receiver via optical (however still a minijack... but don't worry about that... digital is digital... either it gets there or it doesn't... but with fiber optics at least, size really doesn't matter). I would read the reviews on all products first to gauge the risk... On the AirPort Express they are exceptionally mixed - some people love it, and others say it's not worth $5. So YMMV.