I bought my 5670 on Gainsaver and it did not work for me at all. I can't verify whether it was a faulty GPU or that it just wasn't compatible.
There was no boot chime or anything, I even tried a new power supply. I ended up ordering another Radeon 4670 and it ended up working fine for a few days, then it kind of crapped out on me.
I can still boot and I think it actually loads up fine, because when I hit the volume, I could hear the clicks! I'm hoping I just messed up the display connector from disconnecting and reconnecting so many times. I ordered the part, but the only place I could find it was some guy in China, so I've been waiting on it for a week or two now.
If it doesn't work, I've about had it with this thing, and will probably end up just backing up the HD and parting it out.