@joeb
Probably shouldn't have brought up the router comment. It's just something to mess around with. Seems everyone has had a variety of causes and fixes that worked on this thread so there is no.one.answer. A full restore is an easy first step.
I have a similar set up, ripped all my movies to iTunes and share them via a Mac Mini to the Apple TV. I ran iTunes 10.4 for 9 months with no issues. I started having problems after upgrading to one of the early 10.5 iTunes betas. When I rolled back to 10.4 the issues persisted. I did a full restore on the Apple TV, increased my sleep settings to 30min and have not had a problem since (over a month now). I made no changes in my router, computers, or otherwise. I am now using the latest iTunes and ATV betas and it's been stable.
Personally, I think it's either the ATV locking up or a router capacity issue >> my very uninformed opinion and not based on anything other than a hunch. I haven't done enough reading to know which ports or demands Home Sharing puts on a router but router RAM, processor, air interface // bandwidth and firmware have all been sore spots for me in the past. I have a prejudice against certain routers and went with an Apple one, figuring if it didn't work with their equipment, it just wasn't meant to be. The fact that both of your ATV's quit working at the same time would lead me to something they might have in common.
In regards to other media options. I've dabbled with a NAS to boxee, xbmc (on a server and on a jailbroken ATV2), Windows Media and they all worked fairly well. It just required too many steps and changes to be worth my time. Since my issues are mostly gone, I am happy to stick with iTunes and the ATV. If I had to scrap it all and try something else, my 2nd choice would be an Xbox reading directly from the NAS (provided yours supports streaming).
good luck