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I don't even think you can save full retail games on the internal storage. You should have either a 2GB or 4GB SD card with your system from when you bought it and that should be enough, but i can't guarantee it. Considering that there are now several fully animated cutscenes which will be in some sort of CG-movie file format and mastered OST in compressed file formats similar to MP3 files, it's going to be at least 1GB i think. It really depends on how many and what quality the files are in. I think 3DS games range from 256MB to 2GB currently.
If you need to buy a new SD card no matter what, i'd strongly suggest you just get a Class-10 32GB SDHC card , since it's not too expensive and it's really handy in the long run... unless you know you're never gonna download other games digitally.