I have bought my RAM from Macsales for years (their FW drives also) they are a great company and very responsive if there is a problem.
From what I understand –
MacPro certified – same RAM as what Apple puts in
Apple qualified – meets the specs set out by Apple.
I went ahead and got the certified this time for my system just to be safe but I have bought the qualifed before and had no issues.
Don’t even think about getting any other card than the X1900, the multiple card configurations are for a few specific applications and for the video world you are giving up speed (if fact Color will not work with multiple video cards) so unless you really need 8 monitors going there is no reason for 4 cards, but boy would that not be cool!