You can use both. As to SD card speed my buddies have Axims and the high speed cards have no payback. Your choice to spend more for no boost. And a pre-emptive answer. The standard speed is fast enough to play video and music from.
I was wondering if you could use both a Compact Flash Card and SD Card at the same time with the Dell Axim X51v. Or can you only use one at a time. Also I wanted to know what type of SD card to buy for it. Does the tranfer rates mean anything?