The common speed grades come out to 12MB/s, 20MB/s, and 40MB/s, being 80x, 133x, and 266x. Since we want the JackHammer and the NuBus to be the bottleneck, I'd pick the 266x card just to be totally sure.
They're about double the 133x price, about USD80 for 8GB. On one hand, you're spending 40USD for more speed you probably aren't using, but on the other, you already sunk 80 into a SCSI-IDE adaptor }
I guess it's a toss-up, but I have to agree that the 133x card is the better idea.