Something has changed --- either the RENFE site or the FIA card. Last March (2010) I had no troubles at all purchasing RENFE tickets on the web using my Charles Schwab Visa, in two separate transactions. The first ticket was at a >65% discount, the second much less discounted.
btw, when I subsequently called FIA to inform them of our upcoming journies (some weeks after the online RENFE purchases), they expressed some concern about the Spanish part of the trip, but not the Egypt or Jordan parts ---- the CS agent led me to understand that Spain was then a hot spot (with considerable amounts of credit card fraud occurring in some parts of the country).