Does it matter which stick you leave in? In other words, will XP install with either (one) stick in or is it specific to just one of those sticks?
I have read of several instances, where faulty RAM gives problems with XP and W2K installations. For some unknown reason, '98 seems a lot more tolerant of (potentially) faulty RAM.
Just out of curiosity, what happens if you now put both sticks back with XP already installed?