After 6 days of banging away my poor card gets zero credit because it took so long.

The guy who got all the credit for the task uploaded their result a few hours before me.

Is this how it works here? My machine crunches willingly for 6 days and gets nothing?

Just wondering....

http://www.gpugrid.net/results.php?hostid=146218
I think you missed your deadline :/ As you can see the other guy submitted within one day and thus didn't miss the deadline which depends on when you get the WU (is not a fixed date for a specific WU).
But if a WU takes 6 days to finish, your hardware might unfortunately not be suited for this project. I think even the long WU's should take around 10 hours or so on top cards. Maybe you could switch to the short queue.

Someone else will probably answer you soon with more experience on GPUs.

Also in the recommended GPUs: http://www.gpugrid.net/forum_thread.php?id=2507#21073

Your GPU is too slow for long runs and the deadline was missed. The other computer was not issued the WU until the deadline was already past. The deadline is always 5 days AFAIK. Set your project preferences (from your account page) to do short runs only and you should be OK.