Why on earth would you use an Intel 520 120 GB SSD as a cache? The purpose of that SRT is to offer a cheaper alternative to people who can't afford a large enough SSD to install an OS + apps on.
Install the OS on the SSD and use that HDD as a data drive/back up. You'll notice the difference then.
Replacing that HDD with a WD Caviar Black would more than likely even given better performance than using SRT.
Asus P67 Sabertooth | Intel i7-2600k | 2x4 GB Corsair 1600 XMS3| EVGA GTX 460 1 GB SC| Corsair TX650 | CM Hyper 212+ | Western Digital Black 1TB 6 GB/s 64 MB Cache | CM HAF 922