^ what this guy said. most cards ships with at least 2 years warranty.
i get that nvlddmkm error (only in vista) as well. and the only fix i was able to come up with was to completely unplug the system from the wall socket, as a simple reboot didnt help (weird, made me want to split my gtx260-216 in half everytime i see that error). it might help.
I had the same issue with the first batch of 250's and didn't hesitate to RMA the card back to BFG. The replacement card is still going strong 24/7 .....
@ Quicksilver: Just reinforcing what has been said: The card is probably under warranty, so RMA it, there's no need to get a replacement...I hope.
If not then yes, the HD5770 would be a fine replacement although if cash is tight (when not?) and you are not too bothered with DX11 then a 1 Gb HD4870 would probably be a cheaper, and easier to find option.