Thanks to you both for the useful info.
Having reviewed the OpenBSD man page for xrandr, it seems it supports the 1.2 options.
I found a fairly priced dual-DVI Radeon HD 3870 which I'll probably go for - this card has the RV670 chip according to radeon(4). [
http://www.ebuyer.com/product/142605 ]
Cheers...