How much freespace should I leave on my MicroSD

I have an R4 with a sandisk 2gb microSD card (1.87gb after formating IIRC) right now it has 470 something MB left free on it is there any bad effects of filling it too close to max?

Just leave at least 10MB (or 20MB for 2GB) to be safe, for saves when they're created and FAT files increasing.

Thanks, that answers my question perfectly. :) (I'm leaving 50-100MB free just to be safe)

The average save file is half a MB, i believe. If you've already got save files for all your games, go ahead and fill it up. (Each game has it's own save file, doesn't matter if you have multiple saved game on a single cart)

Yes it's 512kb. You can just rename the NDS ROM to the save file to make them work together in loading.