I have a:
Sandisk 8Gb Class-4
Toshiba 8 Gb Class-8
Emtec 8 Gb Class-6
...and I can recommend all of those as "good" SD cards. What do you mean by "lagging" games? DS games are streamed into RAM and played from there, the only lag that may occour is during FMV cutscenes or during loading.