If you change the settings the save sms to the flash drive (like in the s60 phones) then you basically have unlimited memory, BUT obviously this is gonna slow down the phone. On all my previous s60 phones, the phone started to take 3-5 seconds to open an sms message (I had about 2000) but since the N900 has a better processor it shouldn't be much of a problem but that doesnt mean it wont happen. It would probably take a lot more than 2,000 but sooner or later it will slow down. If you really want to save them, just make a new backup when you get to about 5,000, then you'll always have them saved