Doing everything on the very small, antique 80GB drive is not helping you at all.
You should have the OS, applications and application specific plugins on one drive and do all your recording and data saving on a second drive.
If a second drive is say a 2 TB Western Digital Black or Hitachi 7K, you could partition it in half, using the first partition for active tracks and VM and saving the finalized tracks to the second partition.
As for the boot drive (the current 80 GB), new drive models are much faster than that drive could ever hope to be.
Replacing it with a (min) 500 GB WD Black would breath new life into the machine. Installing a 128 GB or, better, 240 GB SSD for OS and apps would give the machine some real snap.
File manipulation would improve due to free space on the drives and should relieve stuttering, but, finally, the current bandwidth and limited multithread capability of the processor will remain.