checking the Samsung Website, they are selling the following and guaranty it to work with the Focus:

This 8GB microSD Memory Card uses the latest advancements in flash memory technology to deliver the smallest memory card in the world. Measuring about one-fourth the size of a standard SD card, the microSD card provides high-performance and expanded memory capacity for your SAMSUNG mobile phone. Package includes a standard size SD Adapter that can be used in your compatible memory adapter device and digital camera.

It is a SanDisk card - they also advertise the 2 and 4 GB.

That being said, other BB's have opined that you need a class 4 or 6 (if available) card so as to not slow down the Focus.