The 450W Corsair a solid PSU and will serve you well but it's not for you. 3A is hardly enough for the rest of your system. The 520W one is much better, since it has three +12V rails with a total of 40A on all of them combined. Pretty good and from a reliable manufacturer too. Also, the X1950PRO/XT is as far as you can go without encountering a bottleneck, so it's a good choice. Good luck and enjoy the new card.