What's the best OS for a 770(Pentium I/II 300+ MHz?) I usually work up to putting XP
If you just install Windows XP, without any Service Pack, updates or virus scanner, XP is actually quite acceptable even for 233MHz Pentium MMX and only 128MB of RAM, i.e. Microsoft's official minimum system requirements for the original XP. I ran XP Pro on an ancient Packard Bell desktop with 233MHz and 128MB for couple years before the Service Packs and the ever-growing virus definitions started to slow things down too much.
As for supporting older hardware, Vista pretty much killed all of it...on purpose, I'd say. What M$ wasn't counting on was the amount of people who decided that they would rather stick to XP than buy new hardware to match the latest OS...but I digress.
Forcing consumers to upgrade their hardware would benefit hardware manufacturers, not Microsoft. I have always suspected that Microsoft had some sort of a secret pact with the hardware manufacturers, which backfired, of course.