Soundcard: I do not have any experience with Gravis Ultrasound. It may be good, but in any case you need some ISA SoundBlaster compatible card (I do not know how it is with Gravis Ultrasound in this area). SoundBlaster 16 worked for me ok when I had Pentium 100.
You may want a wavetable card - it should have better sound - I think these are e.g. SoundBlaster AWE 32, 64. But remember: always ISA, not PCI version.
Operating system: you will use mainly DOS, you can install Windows 3.x or Windows 9x if you want, but you will not find many advantages in using them in comparison to your new computer.
Yes, W9x Windows (apart from Windows Millenium where it is a bit problematic) have good DOS inside. So you can use them as DOS as well (booting just command line). It is possible to make there a booting menu, so you can choose when computer starts whether to boot Windows or just a command line.
Memory: at least 8 MB, better are 16 or 32 MB (you can then run W9x reasonably well and there are some DOS games which can benefit from it - e.g. Blood is happiest with 32 MB). Do not install more than 64 MB of memory - it is unneded for DOS games and some can have problems with too much memory (reporting than they do not have enough memory 😀 ).
Processor: high eng 486 or low end Pentium (up to 100 Mhz). This should run most of games well, just the oldest ones will need slowing down (which is not problem on that configuration or you can use DOSBOX on your new computer to run such games).