It depends what the rest of your system is like and what resolution you play at
Generally, if you get 2 x 4870 you are stuck with no upgrade path. otoh you could get a single 4870x2 and you can later add a 4870 for CF-x3 like i do, or even another 4870x2 for CF-x4 when they get cheap[er]
i paid less for each of my cards - quite awhile ago; look for a great deal ..
and that second card you link to is a 512mb 4870; get a 1GB version for CF if you go that route