Hi Arto,
Try, a 'glint' Permedia3 -driver card, works on the as500, i noticed the 
same problem
with 4.4.0 (pre) using a matrox Mystigue(mga 2064sg), i have a as500 
running nicely with
a 3D Labs VX1 oxygen card , dmesg ->

>NetBSD 1.6ZF (NEW) #1: Sat Nov 15 04:00:28 EST 2003
>	mel@casey219.doodle.com:/usr/src/sys/arch/alpha/compile/NEW
>Digital AlphaStation 500/500, 500MHz, s/n 
>8192 byte page size, 1 processor.
>total memory = 256 MB
>(2256 KB reserved for PROM, 253 MB used by NetBSD)
>avail memory = 228 MB
>using 1637 buffers containing 13096 KB of memory
>mainbus0 (root)
>cpu0 at mainbus0: ID 0 (primary), 21164A-0
>cpu0: Architecture extensions: 1<BWX>
>cia0 at mainbus0: DECchip 2117x Core Logic Chipset (ALCOR/ALCOR2), pass 3
>cia0: extended capabilities: 21<DWEN,BWEN>
>cia0: using BWX for PCI config access
>pci0 at cia0 bus 0
>pci0: i/o space, memory space enabled, rd/line, rd/mult, wr/inv ok
>tlp0 at pci0 dev 6 function 0: DECchip 21040 Ethernet, pass 2.6
>tlp0: interrupting at kn20aa irq 13
>tlp0: Ethernet address 00:00:f8:25:56:55
>tlp0: 10baseT, 10baseT-FDX, 10base5, manual
>eap0 at pci0 dev 7 function 0: Ensoniq CT5880 CT5880-C (rev. 0x02)
>eap0: interrupting at kn20aa irq 8
>eap0: ac97: Crystal CS4297A codec; headphone, 20 bit DAC, 18 bit ADC, Crystal Semi 3D
>eap0: ac97: ext id 200<AMAP>
>audio0 at eap0: full duplex, mmap, independent
>midi at eap0 not configured
>vga0 at pci0 dev 8 function 0: 3D Labs GLINT Permedia 3 (rev. 0x01)
>wsdisplay0 at vga0 kbdmux 1: console (80x25, vt100 emulation)
>
the mga millenium card works in 4.2.1 on this box. E-mail me off list if 
you need
any pointers.
-Mel