I second that, avoid lowcost cards like the GTX550. The GTX560 is the minimum, GTX570 would be better if you can, and look the bench here : http://www.luxrender.net/wiki/LuxMark_Results
And since a few months already, and new improved OpenCL support, the AMD cards have a better price/performance ratio. So a HD6950/70 is more interesting than a GTX560/70, unless you definitely want a Nvidia cards.
But i would advise you to wait as AMD is just going to release its new cards next month (with the new 28nm process).
They will probably be more expansive at first but more interesting on the long run, more RAM, consume less power, probably very good in OpenCL.
And right now hybrid Path OpenCL in Luxrender is not really interesting (at least from my point of view, i am afraid my CPU cannot feed enough my HD5870), all the hope is on hybrid bidir, initial test i did showed a doubling of samples/s. However hybrid bidir is not finished yet and only available on the development branch.
So don't make a too fast purchase, take your time to see benches about the new AMD line of product. And even i you don't want to jump for the new HD7xxx, may be there will be some bargains on the older products.