What is the best kind of memory card to get for a DSi?

I suggest the Sandisk 4gb selling for $19.99 at GameStop
specifics:
When there’s a need for more music and pictures a Nintendo DSi SDHC Memory Card made by SanDisk is the answer. Just insert in a SanDisk SDHC memory card and there’s plenty of space for loading up favorite songs and snapping photos. With two built-in cameras, AAC music playback and only 256MB of onboard storage, it’s easy to run out of room on the Nintendo DSi. SanDisk’s 4GB card can give the Nintendo DSi up to 15 times more space. The SanDisk Nintendo DSi SDHC Memory Card is the must-have accessory to store more on the Nintendo DSi. And with the official Nintendo Seal of Approval, it comes with the compatibility and quality trusted from SanDisk made for the Nintendo DSi.


Best Buy has that same card, but it sells for $29.99!

Another choice is the Data MINE 8gb selling for $24.99
specifics

• 8 GB storage capacity - Class 4 - Guaranteed fast data transfer minimun rate 4 MB/ seconds for reliable copy/download/backup.
• Usable with an extremely wide range of devices that have Micro Secure Digital (MicroSD) compatible w/ media slots ; Includes Mini Adapter and Standard SD Adapter
• Low battery consumption to maximize battery life in small portable devices.
• User-selectable mechanical write protect switch on the exterior card casing.
• *** 3 YEAR WARRANTY *** ***Includes Accessory Bag***

Thank you for allowing IM shopping to assist you in finding a memory card for your DSi game console. 

Attached are three popular options. I would strongly suggest the Scan Disk option because they have been industry focused on memory cards for various mediums for quite a time now. I have never been disappointed with their products at any time.