There is no non-free driver for your GPU for OpenBSD.
hdradeon is not a driver, but radeonhd is. It's open source but only supports x1600 through HD4xxx GPUs. On linux, the radeon driver supports all radeon GPUs, but requires kernel modesetting for acceleration on HD5xxx and higher and even modesetting on HD6xxx and higher.
So, theoretically, you can get basic modesetting working on your GPU with the 'radeon' driver, but you need a newer version. OpenBSD 5.0 comes with xf86-video-ati version 6.12.2, while modesetting for HD5xxx GPUs requires at least 6.13.0.
If you know what you are doing, you could grab
ftp://ftp.freedesktop.org/pub/indivi...-6.13.2.tar.gz and compile it yourself. That will likely also involve upgrading libdrm, though.