Sli = Scalable link interface
CFX = CrossfireX
Nvidia uses sli. AMD uses crossfireX. Same idea but each company has their own method of combining two graphics cards to increase graphic performance. I think you are looking for an Nvidia sli ready card.
In your price range ( up to $170), I think you are looking at either a gtx 460 or Radeon HD 6850. The tom's review says you can get a gtx 560 or a 6870 for $170 but even the low end 6870's at newegg.ca are $180 and the price goes up from there. The 6850's and gtx 460's are rougly $150.