Had to do some researching on Ray Tracing Games. But from articles it would seem that the concept was started with the launch of the 4800 series Launch.
This one particular article refers to the shader power for the 5 series cards playing a lead role in the decision to create this technology. It's an older article but my suggestion would be for you to take sometime and do some researching on your own
The first PC game. With Fusion Rendering Cloud and upcoming OTOY game streaming service, this experience will be available for everyone sometime in 2012
. World's first ray-traced PC game and movie to arrive in 2012 - Bright Side Of News* PC game. With Fusion Rendering Cloud and upcoming OTOY game streaming service, this experience will - Google Search
Their is a lot of information out their to search through. The your answer looking for is very subjective pending on a lot of variables. It's an answer I don't know if anyone can tell you exactly what to expect but with all graphics driven questions it all depends on what game your looking to play.
I would suggest you look for the best DX11 card with the highest number of streaming processors.
Of course this answer comes form just the little bit of information I was able to read through in an hour or so.
I also read an article that Intel had demonstrated this process using 4 servers and and 1 "Knights Ferry" addin card. Although Intel doesn't call these graphic cards, they essentially are. At the "core" there are 32 cores clocked at 1.2ghz each, and a total capacity of 128 threads per card.
Hopefully someone else on here can provide us both with a more definitive answer.