Service Pack 3 won't run some of the games I want it to run.
Ah, that's games older than (or too soon after) SP3 that don't take account of some of the tightening up of the system (or just plain messing about with) that this service pack did, then. And at the same time other, and newer, games that you're worried won't work with XP (or at least XP pre-SP3). Tricky. Bit of a conundrum.
Dual-boot XP(SP2)+XP(SP3)? Is that doable/allowed/practical* as far you're concerned? If not, XP(SP2) plus some later version of Windows that you're happy with. Again, getting that working right might be a bit interesting.
[* - Delete whichever is/are inapplicable]
I think the only way to know is to try everything out. I'd go for checking to see if the SP3-unfriendly games (or particular games which are spoilt by the game-unfriendly SP3, whichever...) have fixes and get-arounds out there for an SP3 environment. That's because while there are some things that I'm not happy with in the whole XP update situation[1], I'd much rather have an XP environment patched and updated up to the nines (the previously noted exceptions... er... excepted) than have it sitting at an SP2 level just because of an individual game whose writers didn't have any interest in granting a solution to get around various changes...
But I feel I'm probably proselytising a bit, there. One way or another, this whole situation might need a bit of re-reinstalling, if rolling back doesn't correct certain things, so tread lightly (but test fully!) before you settle on the solution you want.
Another alternative, dual-boot again, but this time one the fully-updated XP, the other a decent Linux distro with an up-to-date WINE. See if WINE can play anything that the XP no longer can? Again, might need a lot of fiddling under the hood, in some way or form, but it's another option.
[1] Most particularly, if a machine is set up as a UK locale, with a UK keyboard, with UK everything (which takes more trouble than it's worth, and at least one thing good about Vista and onwards is that you say "This is for the UK!" on set-up and you don't have to go in and make sure obscure things like the non-Unicode behaviour is also set to your region, unless you want to make it other than the logically connected choice) and then, because you have absolutely no interest in having a US keyboard layout, you remove that from the configuration and moreover disable the language taskbar icons,
then you install various updates from the SP3-era, if not SP3 itself, and it'll do silly stuff like re-adding in the language taskbar icons! That little "En" icon gets on my nerves. There's a couple I know who have recently come to the UK from Australia and have an Aussie English keyboard (subtly different, but can't remember how, probably the GBP symbol isn't there, though, nor the Euro), and they have the "En" on for much the same reason, even though they'll never need to be using a UK English layout. But I leave it there because it's no my place to make such decisions on their behalf. 'Scuse me, going off on a tangent there.