Quote Originally Posted by Pharnakes
I thought ram has to be absolutely identical to get the very best pairing, so if you buy it all together you will get the best pairing. I have heard of instances were poorly matched ram means that you actualy get a preformance increase if you take some out.
Yes, to be safe it is best to have identical RAM. In my opinion, buying 4 GB RAM now is a better invesment than buying 2 GB now and buy 2 GB later (who knows if at later time, the products are not available? The price goes up? etc)

As for XP-64 bit, it is infamous for being incompatible with many programs, so if I were you I'll go with XP 32 bit regardless of all the advertisements.