YOu have to realize that XWA is more laggy then other games you are used to.
Just make sure that both comps have the same XWA game, with no patches, except the 2.02.
Also, when you are playing Via internet, you shouldn't use Missiles or warheads (inluding flares) this will cause packet loss or those "little blue boxes"
Turn deatail down to a reasonble level, normallyi run with detail to the lowest setting, 800x600 res, 3-D acccel on, and engine glow. That is enuf detail for me, and it makes it easier on the comp.
Try to refrain from having big large face count battles, that is having 48 ships on a side, or Captial ships slugging it out.
cant say much else.. jsut make sure you got the lastest drivers for your vid card... thats all i can say really.... hope it helps