Do You Need a Memory Card to Download DSiWare Games? No, But Recommended

No. The Nintendo DSi has 256 MB internal flash memory (system reserved ~100 MB; ~150 MB user-available). DSiWare downloads store there by default.

When You Want an SD Card

• More Space: DSiWare games range 1–20 MB each; 150 MB fills fast.
• Photos / Audio: DSi Camera & Sound apps save to SD.
• Transfer: Move saves/games between DSi systems via SD.
• Homebrew / TWiLight Menu++: Requires SD for custom firmware.

SD Card Specs

• Standard SD (≤2 GB) or SDHC (4–32 GB). No SDXC (64 GB+) – DSi firmware doesn't support exFAT.
• Format: FAT32 (cluster 32 KB recommended).
• Class 4+ speed fine; no need for UHS.

Management

System Settings → Data Management → Copy between System Memory and SD Card.

Bottom line: Not required to download/play, but highly recommended for any serious use.