Before you go out and "blow your wad" on a Mac Mini just to act as a media server, you may want to investigate why your current machine is behaving like that. I use Windows7 with iTunes without a hitch, no lag and no freezing.
Sounds to me like it's either: 1) running out of memory (should have 2+ GB at least) 2) having network communication issues 3) something disk related.
Are the two AppleTV's connected to the server via wireless network or wired? I wouldn't exclude your router from analysis either - especially since spending 700$ on a new mini wouldn't fix anything in this case.
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