It occurred to me that Wolfgang S. Rupprecht wrote in gmane.os.netbsd.general:
> A cheap pair of cards that seems to work well are:
>
>   Asus ATI Radeon 9200se   (PCI)  ~$50
>   Asus ATI Radeon ax300sex (PCIe) ~$60

One word of warning: The x300 is a bit tricky to get working using native
XFree86. It'll work with xorg, but to get it working with native X11 I
had to do rather nasty things (Like overriding the ChipID, making XFree86
think that it's really an AGP-based radeon 9800-XT (RV360) instead of a
PCIe based x300 (RV370). I fully expect things to explode behind my back.)

Unfortunately, xorg is not an option for me right now, as I can't seem to
get it compiled on my /amd64 box.