What operating system you use?
Vista has Windows Mobile Device Center that will synch with your X5.
You'll need Outlook to synch your calendars and contacts. I am not sure if there's third party software that will do the job.
Here's more information about synching: (and the latest Microsoft Activesync download)
ActiveSync Downloads – Windows Mobile Synchronization - Windows Mobile Device Center
If you use Windows XP or older, you will need Microsoft Activesync software
I think Microsoft Activesync 3.8 works best for X5. You can download this from:
Free Microsoft ActiveSync 3.8 English for Windows Family Download
You can check out drivers and downloads at:
Dell Support - Technical Support, Drivers & Downloads, Troubleshooting, Product Manuals and Order Status
Hope that help start you off. :)