Yep, the slot is for a proprietary 32-bit memory card, basically just adds 8 more SIMM slots. I don't have the card for it, and I'll probably never find one... not that I'm really looking for one to begin with, since the existing 8 SIMM slots are plenty enough.
The oscillator in the corner works just like the oscillator on pretty much every early-'90s board, it controls the CPU speed. To change the processor speed, you change the oscillator. Though one interesting thing about this board is that it has a jumper to switch between 1:1 and 2:1 frequency... so with the 33mhz oscillator in there, I can run it 1:1 for 33mhz CPU speed, or 2:1 for 16mhz CPU speed. (The board officially supports the old 16 and 20mhz 486 chips.)
And the cache chips are socketed... you can't see it too well in the picture, but yeah, swapping 'em out will be no big deal.
As for what I'm gonna do with an ISA 486, I have a few ideas I'm gonna try, but for the most part I'm just gonna play with it and go from there. I might end up setting it up in place of my 386DX/40.
About the SlotA, SavantStrike pretty well has it... SlotA is just something different and interesting. Actually, that little MSI board isn't going to get used any time soon, I just grabbed it since it would've been thrown away otherwise. I already have another SlotA system put together which I use for general Win98 stuff, and that one is a bit better performing anyway since it has a faster CPU and the KX133 chipset (which has 4X AGP and 133mhz memory support).