Counter-question: why do you so specifically want to use a P5A-B if you clearly don't know much about what makes that board unique, and want to pair it with a CPU that won't do it justice?
So, to answer your questions:
Regarding versions, it's mainly about Aladdin V M1542 chipset revision.
- Everything up to and including 1.04 has the earlier M1542 rev E or older, which is limited to 64MB cacheable memory.
- Everything from 1.05 onwards has the newer rev G chipset, which can cache all its memory up to 768MB chipset limit.
But... 1.05 onwards introduces a bug that causes terrible performance with K6Plus CPUs (K6-2+ or K6-3+) with their own on-chip L2 cache. This bug can be fixed with a simple mod, but with a CPU with on-chip L2 cache you don't need the motherboard cache anyway, so it's easier to just go for an older revision board.
With a P233MMX, this is pretty irrelevant as nothing that would run decently on that CPU would need more than 64MB and in fact with DOS and Win3.11 you'd actively get into trouble with old games/applications incorrectly detecting your - from their perspective - huge memory. With that goal, I'd stick to 16MB RAM, which is actively problematic on a board like this.
Regarding fastest CPU: that depends on revision. The fastest So7 CPUs are the K6-3+ CPUs. They were generally sold with clock speeds around 450-500MHz, but almost universally clock up to 600MHz and beyond. The absolute highest performance will be when paired with a late G-revision board that has been modded to allow these CPUs to work properly. Unmodded, you can choose either a K6-2 550 (which will probably also run at 600MHz) without on-chip cache, which will give highest raw performance, or a K6-3 (non-plus) at 450MHz (won't clock much higher) with lots of L2 cache that will beat the K6-2 without on-chip cache in practical use cases. With an earlier revision (<= 1.04) it's simple: K6-3+.
How much faster these will be than a P233MMX depends on what you are doing. Clock-for-clock the FPU of the Intel CPU is faster, but K6-2 (and 3) CPUs clock so much higher that even in FPU performance they will win. In ALU terms, the Cyrix 686MX CPUs (and MII derivatives) are actually fastest clock-for-clock, but with weak FPUs and they don't clock very high. And as all So7 systems are very memory/cache limited, integrated on-chip full-speed cache (on K6-3 and K6-2+/3+) massively boosts performance. Similarly, bus speeds (and thus also motherboard cache and memory speeds) strongly influence performance. But it really differs per application. And it differs on how you clock your CPUs. A lot of P233MMX CPUs will happily run at 350MHz with 100MHz FSB. Clocked like that they are beasts. But if you just want stock speeds, go for a 100MHz FSB K6-2 or 3 derivative.
As for video card, it depends on what you want to achieve. In general, AGP wins hands-down. But this is an early AGP board and there were both chipset driver compatibility issues with some cards (with nVidia chipsets in particular) and power draw issues with others (careful with non-period AGP 4x cards). Ideal pairing would probably be a Voodoo3-3000 AGP, but that's pricey and DOS/Win3.11 wouldn't do it justice. A TNT2 would be a fine alternative. Under Win9x, chipset drivers are potentially an issue, but DOS and Win3.11 don't care - and nVidia chips have excellent DOS VESA support.
To sum up, the P5A-B is a thoroughbred racehorse (and yes, that includes being tempramental and at some levels flawed) of a board, one of the fastest So7 boards out there. You can frequently clock them up to 120MHz FSB and I was able to run my P5A (ATX version of the same) at 133MHz for a while. They are designed to get the most out of the last generation of So7 CPUs, basically the AMD K6-2 series. Even for those CPUs, there are much less finicky boards available (anything with Via MVP3 chipset comes to mind), but if you want to push 700MHz with your K6-2+, little beats a P5A(-B) 1.04...
...and you want to stick a P233MMX on it and run OSs that even it is overpowered for? That's like getting a Lamborghini Diablo for doing a weekly shopping run. Do yourself a favour and go for a nice i430VX/TX motherboard with split voltage support and add a few 8MB SIMMs. Stick in an S3 Trio or Virge (preferably from a good brand for decent analog VGA output) Much cheaper, much easier than the P5A-B. Save that for going crazy with a K6-3 or faster in Win9x (or 2k/XP)
