I still think it would be worth it. It'll still be faster than a HDD, but the drive might be faster than the controller, which is what the cache is for.
I have 2 Seagate 7200.11 7200rpm 500GB drives in RAID 0, and my peak reads were only 140MBps, right below the 1.5Gb/s bandwidth. My Seagate 7200.12 7200rpm 1TB drive peaks at only over 100MBps.
Thus, SSD are much faster than any HDD. There should be a jumper you put on the SSD drive, to limit it to SATA I 1.5Gbps.