Only you can really decide that Depends on whether you're currently CPU limited for anything, I guess. The thread title suggests this is mainly going to be used for day-to-day computing and as an HTPC, in which case you probably won't really notice the extra performance most of the time. What you will notice is the lower power draw, and particularly the improved idle power draw.
Looking at ebay completed listings if you were lucky it could fetch as much as £20 - it was a great chip in its time and still has more than enough performance for general computing. I've still got a Q6600 rig around that, with luck, I'll have up and running again soon