I can't say too much for the Q6600, since I don't have one. Considering its age, I think it is amazing what it CAN do in 2017. However, I see a lot of people recommending it for budget builds, and that just leaves me sitting wondering why... I run a Q9550, and that sometimes struggles with a title or two.
That being said, my Q9550 with 8GB of DDR2 RAM and a 1050 Ti can run the latest DOOM at 1080p Ultra at around 50 to 60FPS, rarely dropping into the 40s. Not as good as a modern CPU, but definitely a playable experience for someone on a tight budget. The only game I've tried, so far, that doesn't run well on my setup is Hellblade. It was playable, but not the smoothest experience. Luckily, I found the game to be boring anyway, so it wasn't a huge blow to me.
After taxes, I'll probably rebuild my system with a new mobo, RAM, and probably that Pentium that everybody raves about for modern budget builds. I'll keep my 1050Ti and PSU, since the whole point to buying those was to upgrade in the future. My Q9550 was just my foot in the door, and is definitely enough to keep me satisfied for a little bit.
My brother in law informed me recently he upgraded it to a Q9650. I doubt it would have made all the much difference to the ageing machine
It very well could have. I built my wife a cheapo computer (all used parts) so she could play Elder Scrolls Online. The idea was for it to play this single game better than her laptop could. Originally it ran a Q8200 @ 2.3GHz, as that is what was on the mobo when we bought it. It performed well enough to play the game, but it would stutter significantly whenever things needed to be loaded quickly (ie, when her character was sprinting). So, just for the hell of it, I threw in an X3323 @ 2.5 GHz (the whole 771 to 775 mod) that I had laying around. Now the game doesn't stutter at all, and plays smooth as silk.
I doubt the Q9650 would necessarily make it much more capable of playing the latest and greatest titles (maybe a small performance increase). But he may have noticed a difference in the games he was playing more frequently. I guess you'd just have to ask him if it made any real significant difference, if you haven't done so already.