I have the Nvidia 6100 motherboard. I tested it in AH before I installed the new video card I bought at the same time. Put at least 2 GB of RAM on the motherboard, the 6100 based onboard video plays AH just fine. Yes you have to turn down the settings a bit, but probably no more than your current FX5700 settings already are. I shopped around and got good deals on my equipment so I upgraded the video at the same time. You can definitely make do with the onboard on this MB though.
And yes the onboard sound works too. I use a SB sound card in mine, but when I tested the system I did it with no add on cards just to see if it would play the game. Everything worked and had liveable frame rates. You can buy it and always add on later as money becomes available.