Cacheable area depends on the memory controller, which is in the motherboard chipset in boards from this era. That's an ALi Aladdin V. That's an interesting case as the answer depends on the exact chip revision. Early revisions A-F can cache max 128MB, G or H revision can cache 512MB. So check the revision of the M1542 chip on your board. It's the last letter of the second-last line of text on the chip.
If you use a CPU with its own L2 cache, it contains its own controller and tag RAM, so it's completely independent from whatever the motherboard does, whose cache then becomes L3 cache. The K6-3 and K62+/3+ can cache up to a theoretical 4GB of RAM. The L3 cache is of minimal impact on performance, but still the highest performance will be where it also can cache everything, so 128MB (if you have an old revision Aladdin V) - unless your software actually needs more memory; in that case you'll be thrashing to disk if you don't have enough. That is much, much slower than even completely uncached memory. So avoid thrashing at any cost.
Realistically speaking, Win98 on a CPU in this speed won't normally be running any software that needs more than 128MB, in fact 64MB was common in computers in those days. Win9x doesn't really benefit from huge amount of extra RAM the way NT-based Windows do.
Regarding voltage, 2.2V is pretty high for K6plus. The board has five voltage jumpers, so there should be 2^5=32 settings, whereas only seven are documented. Looking at the photo of the V7MA on TheRetroWeb I can't make out the voltage regulator, but the very similar V72LA has a Semtech SC1152CSW voltage regulator. Its datasheet is here. It indeed has a 5 bit VID selector with the combinations listed on page 5. Comparing that to the jumper settings, "0" in the VID corresponds to the switch set to "On" and "1" in the VID is "Off". Also the patterns match if reading VID 4-3-2-1-0 corresponding to SW2/4-5-6-7-8, so I'm guessing the V70MA has the same regulator or one with the same operating logic.
In that case, for nominal 2.0V operation, you'd want to set SW2/4-5-6-7-8 to On-On-On-On-Off
However voltage is only part of the challenge to get K6plus CPUs properly supported. BIOS also needs to cooperate and requires patching if not officially supported. You can find a list of supported boards here. No Acer V70MA I'm afraid. Exactly what to expect if you try it anyway is anyone's guess, particularly as this is an OEM motherboard - but at 2.0V it won't harm CPU or motherboard to try.
