It all depends on how many slots you have available. With P55 having 16 lanes available, taking 4 up with the SATA6 card will have some effect on SLI performance, if indeed you have a spare slot to begin with on P55. X58 however has more lanes so SLI/xfire performance will not be affected unless you intend on running 2x dual GPU cards that require the full 16 lanes or indeed triSLI/crossfireX.