1. CF receiver
2. Serial/mouse receiver (cable connects to bottom port of your device)
3. BT receiver (requires CF BT card)
Look for a receiver with a Sirf 3 (or equivalent) chip. Note that the X5 doesn't have SDIO so an SD receiver is not an option. For more info, check out the GPS forums and reviews:
I used the Haicom HI-303S CF GPS Module with Mapopolis software and maps with my Axim X5. I'm selling it for $50 including the maps, since I do not use the Axim anymore. PM me at enracaed@gmail.com if you're interested.