According to this: http://eab.abime.net/showpost.php?p=829088&postcount=1
" The ACA500 is powered by a 14MHz CMOS-68000 processor with 2MBytes RAM "
"The relatively moderate increase in speed is enough to run WHDload."
So probably the onboard aca 500 extra 2mb ram + 14mhz 68000 is enough for whdload.
As for the memory configuration, if things are like toto says, then normally you cant use trapdoor expansion (as it will also sit at $C0000 address) and probably will end up with a 2.5Mb ram amiga (2mb on aca 500 + 0.5 on board amiga). BUT i believe too, that if you mod your A500 motherboard ( http://www.amibay.com/showthread.php?t=4722
) to see the extra trapdoor 512k as chip ram, you can have an amiga 500 with 3mb ram : 1mb chip +2mb slow!