The 4660/4661 has 1 GB of operational memory and 512 MB of channel storage. So there's some value to an SD card if you have more than a couple dozen channels, as most users do. But your Roku is only using about 2 GB of your SD card. No idea why they programmed it that way, but that's all they'll use.
This wiki includes most of the hardware specs for each player Roku has ever offered (note that the Wiki is specific to North American models only, but should be closely comparable to other markets). It doesn't include much info about the various Roku TVs, as they are all built and sold by different companies and might have different specs.
Dan
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