I can't help much on the hardware part but I had an Athlon XP system and Windows XP SP3 running with 3GB, so basically XP will use whatever RAM you throw at it. Going beyond 3GB makes little sense though even if your M/B could handle it. With some of the addresses reserved for hardware, you'll only use a part of that last GB.
Yeah, without PAE (and without hacks, XP has no PAE due to driver issues) you can't get more than 3.25GB even disregarding chipset limitations. 2GB ought to be perfectly fine, as I'll explain below, but if it's not, I'll take 1GB and be happy with it. The kind of things these specs can run won't need more than that anyway, I just happen to have the modules for 2GB handy.
Dear god I hated that SiS chipset. Via was always the better behaved option IMO.
My experience has been the opposite, though I might just be dealing with these boards in old age and the SiS examples that made it to today were always the survivors whereas the VIAs were more common and are failing now. I have had so many terrible VIA experiences that I will go out of my way to avoid anything they make now. I wonder if I'm just cursed -- like I said, anything before 845 for Intel is weird for me too.
I've had 3 separate VIA boards for socket A just be complete turds. Two were KT133A, one was KM400, and I've had a smattering of various other VIA boards for Intel that sucked at least as bad if not more. Usually Apollo something or other. Yuck.
The one nForce board I've had for socket A was completely DOA. That happens to the best of them so I reserve judgement, but the general consensus is that they're overly expensive and nowhere near as reliable as they should be. They run hot and like to pop capacitors.
I've had a handful of SiS machines and my impression of them is that they just kinda do their job. No fuss, but nothing spectacular. That is precisely what I want from this. So what if it's 10 percent or so behind an equivalent VIA config? At least I'll (hopefully) be able to rely on it continuing to work for the forseeable future.
I had several Athlonic XPs, 1800, 2200, 2600 something like that. I don't recall the VIA more than 1.5GB, but maybe near the end more was possible and I just didn't want to change the mainboard. The Penitum 4 solved many problems for me with 2GB and didn't destroy anything attached. I avoided AMD for nearly 20 years due to the destruction, then bought that slowmo 3950X.
I've had several myself, a 1500+, a 2400+, a 2800+ and a Sempron 3300+ which is just a Barton 3200+ rebadged. I have a mobile 2200+ chip to use as a fallback if the Duron doesn't pan out, but I'm kind of secretly rooting for that little booger. The Athlon XP is frankly kind of boring, the Duron is an underdog and I want to see it punch above its weight.
1.5GB was a common amount for SDR and earlier DDR setups, but a lot of the documentation was made before 1GB DDR modules existed. I've had luck with pushing it before. Yes, the Pentium 4 had a much more sophisticated thermal management solution, AMD just had a thermal diode from Palomino through Barton and relied on the motherboard to sense a problem and cut power where Intel could flat-out scale the frequency on a desktop chip to keep it from killing itself. That was a solution AMD would implement the very next generation, though notably there were some models of K8 chips that didn't implement Cool'n'Quiet too.
Essentially my goal for this build was to end up with something cheaper than Tualatin (look up the going rate for a TUSL2-C and tell me that I'm wrong for spending 50 bucks on that ECS board instead) that could drive my Ti4200 better than it. The whole point of the build is to let that card stretch its legs, I have a strange soft spot for DirectX8 cards and the Ti4200 in particular as it was so ridiculously better than the card that "replaced it" in their lineup for the FX generation. Seems like anything FX is cursed -- nVidia FX was loathsome, AMD's FX line was bad start to finish -- the only things I can say were cool were the Athlon FXs on 939.
From what I can gather, mobile Morgan is completely multiplier unlocked, and before I even get started messing with the multiplier I can shove the FSB up to 266 since it is nominally 200MHz on pre-Applebred Durons. That'd get me to 1333MHz right off the bat, and I'm willing to bet it'll be binned well enough that it'll do that easily. It's a late-2001 part rated for only 1GHz -- frankly, I don't believe that for a second. I'm hoping for 1400 or even 1500MHz from it while still keeping the wattage below 50.