Below are some numbers from a similar system after changing the CPU and the VGA as an indication of what to expect in Quake. I used Phil's benchmark suite for this, you can also check there for more results and draw your onw conclusions. You could try running the benchmark too and see if there's something wrong currently and have some numbers to measure improvement after upgrading.
Phil's Ultimate VGA Benchmark Database Project
My summary, assuming you decide to improve this system: Don't expect miracles. For what you are targeting, DOS software rendered games, first of all get the fastest CPU your board can handle. This will have the greatest impact and then a faster VGA will help, but not that much unless the existing card is really slow. Anyway, I got my 233 MMX recently for ~5 euros from ebay without researching too much and the G450 (not the fastest, but still a really good DOS performer) for ~15 so it shouldn't cost you much to change both.
system / doom fps / quake fps
MMX 166MHz, 64MB RAM, S3 Trio 64V+ 1MB / 71,5 / 36,2
MMX 233MHz, 64MB RAM, S3 Trio 64V+ 1MB / 80,2 / 42,9
MMX 233MHz, 64MB RAM, Matrox G450 32MB / 83,3 / 44,8