I think your post probably would get more views if it was put in this section.

General Old Hardware

Perhaps a moderator will move it.

Have to say though your wishlist combined with "Not Expensive" seems unlikely to me.

If you want 3DFX without emulation then that means you will need a genuine 3DFX card. Voodoo 4 and 5 are very expensive. Voodoo 3 is probably the least expensive for the performance, though still not cheap, and it will struggle with late Win9x games. You can't go too new for motherboards with a Voodoo 3, as it's the older 3volt AGP standard. Pentium 4 motherboards tend to have only 1.5volt AGP, which isn't compatible. If you get a DirectX 9 card though, you can use a wrapper like nGlide that uses DirectX to accelerate Glide titles.

If you want flawless DOS compatibility then you probably need an ISA soundcard. Motherboards that have ISA will be Pentium III at most (apart from a very few specialist Pentium 4 boards that cost a fortune), which will mean late Win98 games may not run at 60FPS. Especially not if combined with a Voodoo 3 3DFX card.

I'll leave the recommendations of what to buy to those with more experience than I. But I'd suggest looking at the youtube channel "PhilsComputerLab" if you haven't already. He offers many suggestions for building Win98 and DOS machines from readily available and not too expensive parts. Including Athlon 64-based and Pentium 4-based. Using cheap video cards like the Geforce 4 MX Series and Geforce FX series GPUs, and using cheap soundcards like the Soundblaster Live.

It is not possible to build a single machine that will play all DOS and Win9X games though. The advance of hardware in that period was just too fast to make a PC that could do it all. If you want a Win9X PC, then I think it will only support fairly late DOS games. It will be too fast for the older ones.

Above sums it up pretty well.

First off decide which is more important, Dos or Windows.

You can still get P4 and motherboard's using say the 865 chipset for a fair price and have native reliable Win9x support. Lower spec Geforce 4 or FX cards are also fairly cheap and have good dos compatibility.
Sound Blaster Live cards are cheap and common, perfect for windows and can emulate a SB16 in dos, although not perfectly.

Thats a cheap build that doesn't compromise too much for Windows and gives basic dos gaming ability.

If you want perfect dos compatibility, then you want something with at least 1 ISA slot but the price will jump up, also for a good speed CPU to match whichever motherboard you decide on.

Anything 3DFX is expensive, even the undesirable Banshee or Rush cards and as already stated will probably struggle to hit your 60fps requirement, 30fps was acceptable back in the day.

If you have room, and want DOS and Win98, I would say have two PCs. One for DOS with a Pentium MMX and ISA soundcard. It can be slowed down to 386 speeds to play DOS games from early 90s and is fast enough to play most DOS games at high settings. Then another PC for Win98. Presumably as fast as possible to get the framerates you desire. Geforce FX is the newest you should go GPU-wise, as beyond that compatability is poor.

Is there any MT-32 emulation that even runs on Windows 98 or DOS and would it even work in a DOS windows under Windows 98?

Probably older Win32 versions of MUNT when it sounded much worse.

Even emulation aside, it will be pretty challenging to get a setup that can run fast enough to run late Win98 games at 60fps but also slow enough to run MT32 games.

If you want to be able to go low enough for speed-sensitive MT-32 games, the fastest CPU you can go for is a Via C3. That will let you run Windows 98 itself well enough, but even around 1GHz will be slower than a P3-500, so those late Win98 games (Unreal Tournament) will struggle.

Conversely, a system that could run the latest Win98 stuff at high frame rates (eg early P4 or Athlon XP) can probably not be slowed down beyond 486 level (which is way too fast for most MT-32 stuff). So I'd say that your requirements are too wide at the moment. I'd choose one of these two, depending on the stuff you want to play.

The good news is that C3 and P4/AXP systems are relatively cheap. I recently built both, a C3 system based on an EUR 25 Via Epia board and an i865 system with early Core2Duo (fastest supporting Win98 fully) for the same price for motherboard and CPU.

This thread from October asked the exact same thing with the way too wide types of games asking to be ran (MT-32 through late Win9X). Advice on retro gaming Windows 98 build

The first page has the same information you've gotten so far but goes much more in depth. I also recommend giving the full thread a read to see where the journey ended up. End of the story a specific period of hardware was targeted and the machine runs just like that hardware and the performance isn't great for later games.

For most DOS and Windows 9x games from 1987 up to 2001, I would suggest a VIA C3 computer similar to my signature except using a motherboard with official support for these CPUs and preferably if it has one ISA slot.

If basic SB16 and AWE32 compatibility is wanted, then look for an AWE64 Value sound card which can be still found cheap. The MPU-401 UART interface (albeit not perfect) is accessible thru the gameport, where something like this can be done without spending a lot of money.

As the others mentioned, you won't be able to achieve 60+ FPS on everything as that wasn't a thing back then.