The Nforce4 chipsets allow cross controller RAID. From the nvidia webpage:
NVIDIA RAID
* RAID 0 disk striping support for highest system and application performance
* RAID 1 disk mirroring support for fault tolerance
* RAID 0 +1 disk striping and mirroring support for highest performance with fault tolerance
* Disk alert system provides a visual indication so users know exactly which hard drive to replace during an array failure
* Morphing allow users to change the current state of an array to another using one single step without reboot of the PC
* Cross-controller RAID uniquely supports both SATA and PATA disk devices within a single array
That BFG board should do what you want. And there is no point in adding a 300gb drive to a 2x100gb drive array. Most RAID controllers require the drives to have the same capacity, and if they dont, the whole capacity of the hard drive wont be used. In your setup, you have a 2 x100gb RAID 0 array. Adding a 300gb in the future, I believe only 100gb of that 300gb drive will be used. The preformance increase isnt great enough to lose 200gb worth of space. If I were you, go ahead and RAID those 2 100gb drives, then in the future get a larger hard drive for backup and storage, and leave it as a single disk.