the mainboard's cache will become L3 cache if you use a cpu with L2 cache on-die (K6-III, K6-2+, K6-III+). so if you really want to get a K6-x+ the onboard cache is of lesser importance. K6-x+ BIOS support isn't necessary in the first place because we have Jan's expertise in BIOS patching :) and as edwin already stated: anything with a VIA MVP3 is fine. i would also consider the ALi Aladdin V as it's nearly on par with the VIA MVP3.
i just list AT boards here, as i think you might use your old case:
FIC VA-503+ (VIA MVP3)
ASUS P5A-B (ALI Aladdin5)
Enmic NMC-5VM5 (VIA MVP3)
Soltek SL54U5 (VIA MVP3)
Commate S7AX (ALI Aladdin5)
Gigabyte GA-5AA (ALI Aladdin5)
Epox EP-58 MVP3C-M (ALI Aladdin5)
FIC VA-503A (VIA MVP3)
Shuttle HOT-591P (VIA MVP3)
PC Partner VIB878DS (VIA MVP3)
Matsonic MS6260S (ALI Aladdin5)
some PCChips boards
far from being a complete list, just some nice boards. take a look around at the huge online auctioneer and return with some boards you might want to get (PM them..)
Anything with an MVP3 or MVP4 chipset.
MVP3: yes. MVP4: no. As a gaming rig the Trident Cyberblade i7 integrated in the MVP4 is just too weak. Additionally an AGP slot is missing due to the onboard vga. the MVP4 however is really great for router/fileserver in SOHO/home networks or as a poor-man's typewriter.