while never mentioned, ut3 can run a lot faster on an i7 compared to an i5, though part of that may also be due to the extra L3 cache: https://www.youtube.com/watch?v=ZXfPPdLEa2s&t=139s

re5 dx9 saw a small uplift in the benchmark with HT enabled when i tested it a long time ago, maybe up to 10%. these were two popular engines so other games may have benefitted as well.

for ut04, i think one of the guides put out a rumor that it was patched at one point to "use one hyperthreaded core" - calling it a rumor because i've never seen any official confirmation, but i guess the idea is they put things like sound on a second thread. in practice what will happen on multicore CPUs is the scheduler will bounce the main thread around, causing a microstutter in theory when that happens, so launching it with fixed affinity to 1 or 2 threads is supposed to fix that. however that only ever reduced performance on intensive maps, at least from what i remember testing this. not showing any benefit or even slight regression with HT can certainly happen in any case though, even with much newer multithreaded games, this perhaps depends on the workload.

one other thing to consider: IIRC nvidia switched from a hardware scheduler to a software one starting with kepler and at some point also optimized their drivers for multithreading, at least for DX11.

as for your assumed 2gb limit issues, would monitor the game process memory usage with afterburner before making a judgment on that.

Multicore CPUs running XP/old games has been beaten to death but i can't find much info about hyperthreading in this context. I know it is useless but i wonder if there's actually anything to benefit from either leaving it enabled or disabling it. Please chime in!

IMHO Hyperthreading was useful. Had it enabled on a Prescott build when I ran Windows XP.
Hyperthreading allowed using the processor pipeline more efficiently and prepared developers to upcoming dual-core processors.
Windows XP itself had 2 CPU cores with either technology, which helped the Windows scheduler.

About same way Win32s for Windows 3.1 wasn't quite the useless joke people used to make fun of, I think.
Like hyperthreading, it allowed developers to work with a given technology yeaes before it hit the market.
Win32s (-as a testbed-) was available in 1992, Windows NT 3.1 in 1993 and Windows 95 (aka Windows 4.0) in, well, 1995.

Edit: For Windows 98SE, HT should be left disabled, maybe.
It's less confusing to it if the PC is an ordinary, non-SMP version. Standard UniProcessor PC, so to say.

Edit: About the micro stutter issues. Good point. Depends on the games, I think.
What I like about having two logical CPU cores or processors in Windows XP, is, that an application or game can't hog them both.
On a true single-core system, the application can hog the system so much, that the operating system goes down its knees.
Windows 98SE, for example, despite preemptive multitasking (for Win32 and DOS applications only).

PS: I'm not so much of an gamer, by the way.