486DX266 is *the* 486. If you want a faster machine I would just get a Pentium or similar. (I know that some people here try to build very fast 486 machines, but that's doing it for the sake of it.)
The machine should have a CL-GD5424 chip. That chip is VLB capable so it is quite likely that it is connected over VLB, which might make it potentially faster than any ISA card.
The benchmarks on http://thandor.net/object/645 are promising. (Please only regard the results for DX2/66.) CL-GD5424 is always faster than Diamond SpeedStar 24 which is based on Tseng ET4000, one of the fastest ISA chips. Some VLB cards are faster, but it does quite well.
I definitely wouldn't use an ISA video card. It might be possible to upgrade the video RAM, though. Are there any spare sockets?
I can't find any information about RAM, but my guess is that it isn't proprietary. 8MB is not a bad start, though - should be OK for many games suited for the system.
Hard drive: There is certainly a limit regarding the size, but usually this can be worked around with drive overlay software.
ISA slots: You should be able to install an ISA sound card. Keep in mind that AWE64 is a PnP card. The machine probably won't have a PnP BIOS so you'd have to use Creative's tools to initialize the card.
Also, if you want to use a separate sound card you should either disable the integrated sound in the BIOS (that won't happen automatically), or you have to set the separate sound card to other resources than the integrated and tell games to use those resources. It might also be possible to change the resources of the integrated sound (jumpers or BIOS setup) if you can't disable it which would be better because then you could use common values for the resources of the separate sound card.
Still, the ESS688 should be a good match for the machine, AWE64 sounds less like a true OPL3.
(Can''t see the photos. The forum you linked to doesn't seem to allow that.)