Buy X2, but since you're indicating you already own and use X1, just keep using X1 until you upgrade to Windows7, which I profusely recommend asap, for better stability and security, but mostly, so that you'll get some real help from the forum if you have issues. I only have two pieces of hardware that are no longer supported in Windows 7 64-Bit: And EWS 88/D, which was a great product, and an Apogee mini-DAC, which Apogee simply outright refuses to support its USB capabilities.
If you have something keeping you from upgrading to Windows 7, aside from files and such, for example, old programs that don't affect Sonar's use, then I'd recommend a Dual-boot machine: Repartition the drive AFTER backing all your files up etc., install XP first, then install Windows 7 on the other partition, so that it will give you Dual-boot choices. I sis this for my transition. Good luck!!