A3D 2.0 is superior to EAX, but many more games have support for EAX than A3D. I have both in my Win98 machine and I have a cable running from the optical out header on the A3D card into the digital-in CD-ROM header of an SB Audigy. That way I just need to adjust which card is outputting what without having to worry about additional noise. I do have a Live Drive I also use with MIDI modules so it makes sense to have the SB for the final card providing output. I used to have an SB Live and had the A3D card running to the digital-in headers of the Live Drive, but upgrading to the Audigy broke something with how it's handled, so cd digital-in is A3D, while the Live Drive's digital-in header is the CD-ROM.
I recommend checking the prices of the SB Live vs Audigy, they were almost exactly the same when I was looking at them. The only gotcha with the Audigy is if you need the game port, the Audigy has a firewire port on its back, and has a header for the game port.
I also recommend thinking about getting an SD to IDE adapter. This made things so much easier. SD cards are dirt cheap, and once you have it formatted I can easily switch it to a modern PC to transfer files. I also used this as an easy way to wipe out a Windows installation and start fresh. Just keep the boot files, and Win98 installation media in a directory. This helped a lot as I was swapping out drivers for different versions trying to get the correct version and install order to make the multiple sound cards work.
The main system is an A64 with a VIA VT8000 chipset. GPU is a new FX 5500. The CPU, MB, and GPU are all post-98 hardware, but are compatible. This lets me run drivers that were developed later and the system so far has been very stable. The CPU supports Cool 'n Quiet which means it has a BIOS unlocked multiplier so can slow it down without needing to open it up. It's not great for older DOS games as it doesn't have a real OPL, but with the speeds, I'm only interested in running late-era DOS games which generally moved over to MIDI and CD Audio. I have a P233-MMX with an SB16 and Yamaha 719 for speed-sensitive games and max compatibility with DOS audio.
After that wall of text, my advice to you is to first come up with a rough budget for your build, that will help make the part decisions easier. It doesn't make sense to get a Voodoo or A3D card that have gotten expensive if you have a lower budget. Next, what era of games do you want to play? Are you OK with only playing DOS games from the 9X era, or are you trying to build a difficult all-around build?