Honestly I'm amazed yours lasted this long. I had a fourth-gen 40GB iPod back in The Day and it was easily the least-reliable Apple product I ever owned due to that internal 1.8" HDD. Mine was replaced five times under AppleCare.

When that eventually ran out and it died again (in 2009 IIRC), I swapped in a CompactFlash card using a cheap adapter from DealExtreme. Once you do that or an SD card mod (more popular these days), have iTunes restore the partitioning scheme and OS and it'll be good to go.