I've been tinkering with a similar setup for quite a while now for the similar purpose, so I'm going to share my experience with you.
The multipliers are locked on all Pentium 3 and Celeron CPUs, so the only methods of slowing them down are as follows:
1) Changing FSB frequency to lower values - ideally from 133 to 100 and 66 Mhz. Older DOS games work differently with L1 Cache disabled on different FSB frequencies;
2) Disabling L1 Cache on-demand - SetMul utility works like a charm, and you don't have to reboot and enter the BIOS setup to do this;
3) Using the chipset throttling capability via the Throttle.exe prorgam. It triggers the chipset to send a special signal to one of the CPU pins with a defined rate, and it skips certain amount of active cycles while remaining on the same physical frequency. The slower the processor - the better and smoother the slowdown is.
For example, if I want to make my 600/133/256 Pentium 3 to behave like a Pentium 1 - I switch the FSB to 66 Mhz (making it operate at 300 Mhz) via a mobo jumper, and then I load Throttle.exe with the "4 -c" switches (-c means keep L1 enabled, and 4 means skip 50% of the cycles) and have 150 Mhz effective frequency - which behaves very close to my original Pentium 1 according to the feel and benchmarks. If I want to play Wing Commander, Test Drive III and other 386-oriented games - I just lower the FSB to 66 or 100 and switch the L1 Cache off via SetMul. And If I want to go even slower (Digger, Defender, Battlezone - the XT stuff) - I go 66 FSB, disable the cache and set Throttle.exe to 7 (87.5% active cycles are skipped). And they all run nice, smooth and correct at least on Intel motherboards with this method (I'm waiting to receive a couple of VIA S370 boards to run some tests), and Celeron 800 (the one with 100 FSB and 8x multi) has worked for me, too.
Here is an example of Ultima 7 performance with FSB66 + "Throttle.exe 7" (as you may know, this game forces the L1 Cache to get enabled, and is one of the harder ones to get working properly) on my PC:
https://yadi.sk/i/A_EPVxrG3VL7Ed
I'm not sure if you'd get the same performance or smoothness with faster CPUs, however. Even the Throttle homepage says 800 Mhz or slower CPUs are recommended to get the best results.
Regarding the sound in DOS games - there are two routes:
1) Find a motherboard with the ISA slots;
2) Find a combination of PCI cards that outputs sound in all the games you're interested in (as no 100%-compatible single PCI cards exist).
For my project I chose to go with mATX motherboards (because of the extremely compact case), so I went the second route, but if the board size is not the issue - go the first route with ISA sound cards; your best bet is probably a later 440BX motherboard like the ASUS CUBX-L.
If you need the 2003+ performance without the slowdown drawbacks of a faster Pentium 3 - you can also go with a 478/775 Pentium 4 or Core 2 Duo CPU with an industrial ISA-equipped mobo. PARUS has a lot of experience on these:
CPU Tuning, Throttling
