Sure - theoretically, the fastest spinning drives might saturate a SATA 1 bus, but in practice you'll still get significant speed increases, not so much in sustained transfer speeds, but in seek times and perceived snappiness.
The performance will be far better! Obviously the read/write speeds will be nowhere near where they should be, but it will be a worthwhile upgrade.
Difference between the standard drive and the (ancient) G.Skill Falcon II in my 2007 was like night and day.
Sure - theoretically, the fastest spinning drives might saturate a SATA 1 bus, but in practice you'll still get significant speed increases, not so much in sustained transfer speeds, but in seek times and perceived snappiness.
i say go for it - if you are keeping the system long enough to make it worth while. I put an old intel 320 into a 2006 mini and its faster. In your case, youre also getting less heat and power draw in the bargain. And its not like its a major investment!