With a 5400 RPM drive, another thing you may want to try is adjusting Disk I/O buffers in SONAR preferences. If the buffer is too small or too large, it may not be feeding the audio engine properly.
If you open SONAR Preferences, check "Advanced" at the bottom, and look at the Audio->Sync and Caching page. In the "File System" section there is a Playback and Record I/O buffer size (KB). IIRC, they default to 256, and you might want to try two things: 1) bump these up to 512KB each, and 2) enable Read Caching and enable Write Caching (I think these are off by default).
The caching will also allow for the system to be used as a cache if the handshaking to the drive cannot keep speed.
Another thing that will improve seek time on a magnetic drive is to defragment them regularly. They tend to write in any available space, but then reading makes the heads move more than they need to (a single file may be in numerous chunks, each with a "12ms seek time" to recover). An "optimized" defragment will also consolidate data files together, but this is not required as frequently - these also take longer to perform, so best to start when not using the machine and walk away (don't sit and watch it).