Sse3 came out in 04. So it's likely that both 2000 and XP support it but I'm not sure about 98se/ME. I tried looking it up but got a ton of conflicting information.

That's what I was thinking; I'll most likely go with 2000.

Windows 2000 on the 775 platform just absolutely flys it's just stupid fast. You'll enjoy it.

And unlike Socket A, I can actually use a modern power supply due to the presence of a 4-Pin 12V CPU power connector.

True but there are a good selection of socket A motherboards with the 4 pin cpu connector

The actual question is what software you plan on using that uses these extensions? If the software lists its requirement as Win2000 or WinXP, that is a pretty good clue what you should do 😀

If you are really meaning just 'what's the oldest OS', I mean yeah, the only benefit is whether it is aware of the extensions for software that uses it. I don't think you'd really gain any intrinsic benefit on a piece of software that runs both on Win98 vs Win2000 on the same machine, even if Win2000 is aware your CPU has SSE2/SSE3.

I probably should have been more specific in my question. I was just curious if Windows 2000 supports SSE2/SSE3, which it most likely does. I've heard that Windows 2000 is slightly more compatible with 9x era games than XP is, and I want to play 9x era games and 2k/early XP era games on the same system, around 1995-2004 or so. I wouldn't even consider running 98SE without an ISA slot for DOS sound, and I don't need DOS compatibility for this system.

I've had great luck with playing 95/98 games on windows 2K. There are a very small handful of games that don't like 2K but it's rather smooth sailing

There is no specific support integrated into Windows for SSEx. You need software/programs using SSEx instructions and being compatible with a certain version of Windows - so it just depends on the software you're using. Maybe there are some Windows components (f.e. Media Player/codecs!?) with support for SSEx.

kind regards
soggi