As Bootlegninja said, check out a nintendo-based forum.
But to quickly answer your question:
Get an Acekard 2i, EZ-Flash Vi or better a M3 Zero. I can't recommend any cards with "r4" in it's name. They are all rip-offs of cheap existing cards.
Please note, none of the cards have been confirmed to withstand future firmware upgrades (the M3 Zero seems to be OK, because Loader and Firmware is upgradable) and none of them launches DSi-Software because there is non yet.
Slim PS2 SCPH-70004 (v13) w/ FMCB, 3x Sony 8MB, 1x MaxMemory 16MB (Backup card)
I'm not available for FMCB installation service until I can guarantee a processing time of a few days again, sorry.