You know the onboard S3 Trio is very similar to the S3 Virge based Diamond Stealth card right?
Performance wise they will be very similar but the Virge does have 2 benefits
-Looks like onboard is still only 1MB. 2MB will allow for some higher resolutions in Windows, more useful for Win9x though
https://www.danielsevo.com/bocg/bocg_screenres.htm
- The Virge will also allow you to play S3D games which this PC is a good match 3D Accelerated Games List (Proprietary APIs - No 3DFX/Direct3D)
But for me the 2 are too similar, I'd either go back to onboard or use another card, maybe something like a Matrox Mystique.
This isn't so much an upgrade more to justify not using the onboard chip and that's 100% personal choice.
Add a Voodoo 1 card for glide games?
External Midi? the CT2970 does have hanging note bug, but try it out before worrying about it. I've a Roland attached to a flawed card and and rarely notice the issue.
None of my suggestions are must have, I'm just trying to spend your money 😉 after the CPU upgrade it's a damn fine PC as is