You have to add services to deal with IPX, TCP/IP is default in XP. AFAIK IPX isn't available in the 64-bit XP edition, which makes me think it will be gone by Longhorn.
From what I understand, and I could be wrong, IPX is more unreliable because no ACK's are sent; meaning that the sender never acknowledges that the packet it received by the recipient, as is done with TCP. Also, IPX is chatty, sending tons of packets into your network, which can slow your network down because of packet collisions. I have no statistical evidence of this, but I'm sure there is some.