Yes 6xx0 cards work with the Sam440. Amigaboing benchmarked compositing on a 9250 vs a HD6450 in this video.
There is still no W3D or Nova driver for it.
If you can get access to one of those MSI cards with the BIOS switch it could be worth testing but I wouldn't suggest you buy one unless it's super cheap or you have another system it would be useful in. They're all full profile cards so you'll need a PCIe flexi riser so you can run it outside the tower.