I have a 256 MB Viking CF Card I bought from Buy.com awhile ago (for about $20 more than it is now

) and no problems . I bought Viking because I have a 128 MB CF card in my digital camera. I am happy with with both. The deal on Buy.com for the 512 MB is a great buy. I also have a 64 MD SD card from PNY and that hasn't given me any problems either.
The CF card actually sits below the top level of the Axim. The Sena case shouldn't be a problem. I'm waiting for my case too!
Hope this helps.