Well, either nobody was interested or nobody knew the answer.
So I bought 2GB Ram and replaced the 512MB to experiment.
The difference is dramatic at the Apple TV2's. Album Art is more or less instant even when being accessed for the first time.
No problems with streaming 720p movies or anything else using Ethernet throughout.
So if anybody is interested in using an old computer as a permanently turned on Home Sharing it works very well as long as you max the RAM. It doesn't have to be a powerful computer.
