The 5000 series cards sometimes had some VRM overheating issues that caused the drivers to crash when it occured. It's probably something releated to that.
I've not heard any issues with any 6000 cards with that issue.
A 6850 would likely be a good choice for you: http://www.newegg.com/Product/Product.aspx?Item=N82E16814125348&cm_re=6850-_-14-125-348-_-Product
I have not heard anyone having those issues with it, and Gigabyte is a pretty good card. Free shipping too.
A GTX 460 1GB would also be a nice card. It can't be SLI'ed on your board (assuming you have multiple PCIe x16 slots), but you can indeed run an nvidia card on your board.