I suspect any issues would depend on the actual number of books or their total size, rather than the capacity of the card itself.
I have an 8GB SDHC card in my 505, dramatically underused with a couple of hundred books on it, and it works fine.
In my case, I'm pretty sure the limiting factor will be (as HarryT says) the current cataloguing system - it will get unusable long before the card gets more than a fraction full.
/JB
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