PS, edit: ** Ah forget it, this is an AGP card, HyperMemory and TurboCache have nothing to do with it. And AGP Aperture size won't matter here.**
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Don't worry about Hypermemory or TurboCache, it doesn't steal memory, it's like AGP Aperture it set aside a dynamic portion from 0 to it's max (some cards can access 1+GB), but it only uses it when need and when the drivers think the need outwighs the system using that chunk. It's very effective, and compared to using HD space or simply baulking, it's good to have. But for just running Aero, I doubt any card would go to HM or TC for large amount of resources. The only one that might even access it would be the rare 64MB GF6200s and X300s, and then it would likely be just another 64MB if that.
I'd say get at least 128MB, but I wouldn't worry about HM and TC in general, the performance difference will be nada for this type of app. Gaming though is a completely different story.