stock, it wont change

And for cooling which is better? Tall passive heatsink or low-profile active heatsink? I use standard metal thermal grease

For sound card, which is better? ESS AudioDrive ES1868F or Sound Blaster ViBRA 16S?

How much I need memory for gaming? 16MB (stock) or upgrade to 32MB?

Is Intel 430FX supporting Pentium MMX? (oops, typo)

Is 3dfx Voodoo1 supported on this machine?

active > passive
probably the ess, but its not much of a choice
more is always better
usually
as long as you have an open pci slot, yes.

I want to installing CD-ROM there, however to installing CD-ROM I should using drive rail. I don't have drive rail, what can I do?

Should I use DOS 6.22 + Windows 3.1 or Windows 95?

And which is better? S3 Trio 64 or Trio 64V2/DX? I own this video card but I don't have Voodoo1 🙁

I say Win95 OSR 2.1 or 2.5
Trio 64V2/DX is better, of course.

Err... Something is missing:

According to HP Vectra Familiarization Guide for processor upgrade:
"The 90 MHz, 100 MHz and 133 MHz processors need a passive shortening
block VRM (providing VCC voltage), while the 120 MHz processor needs an
active VRM (providing VRE voltage). The appropriate VRM is provided with
each processor upgrade kit."

I was hesitant when I just reading this. Should I need active VRM or short block VRM?

And where I get the rail kit for installing CD-ROM?

I'd try it with the regulator installed. then without. Your motherboard may be the case where it will take a 200mhz cpu but will NOT work with mmx. it happens. Before you get too deep in changing jumpers take a picture or make a paper diagram. Its saved my arse more than once in the past when an upgrade refused to work and I forgot what the settings were initially.

if the rails are NOT in the case already, your only hope is finding another hp vectra machine and taking its rails.