Ran into that maximum pre-rendered frames/frametime spikes thing you mentioned earlier on this topic, intended to start Thief II (with NewDark) and right away noticed something was off. Just looking at the manor right where the first level starts felt like the game was running at 15fps. Hopped back to desktop to check was there something funky going on in the NewDark config file and immediately remembered your posts about frametime spikes when I noticed a setting for limiting prerendered frames to 1 in there.
Started up RTSS, booted the game and the frametime graph looked like damn seismograph. Setting max frames to 1 through the conf fixed it completely, massive improvement. Then I went to check the NewDark installs of Thief 1 and System Shock 2, Thief 1 already had that setting on but I think it was flipped on by TFix which I've also installed since both Thief 2 and SS2 had it off by default. Implying the author of TFix felt this is a common enough issue to just have the patch enable it.
System Shock 2 also had fluctuating frametime, but it didn't feel anywhere near as bad as Thief 2 did. Hell, I even completed the game last winter never noticing anything about the mouse feel. But after flipping that setting on I did notice mouse movement got snappier.
Aww man, now I'm all paranoid and probably end up testing every game with RTSS. And I thought I was sensitive enough to not need any tools to tell me is the mouse as snappy as it should!
Also to your post above, my own experiences with a similar system are very alike, compatibility with WinXP+ games is high. From the top of my head I can add only Red Faction (2001) which needed a community patch to fix issue with too large VRAM and Medal of Honor: Allied Assault (2002) which needed Win98 compatibility mode and manually killing MoHAA.exe in task manager after quitting the game. Also my success rate of getting Win9x era games to work has been like 99% with community patches and wrappers.