Why replace the oxygen sensors if you're not getting a CEL? Is your fuel mileage down?

I'm on my original water pump as well, although I do have the cooling system flushed every 100,000 miles. My first set of sparkplugs lasted 126,000 miles before a misfire was recorded, and it still wasn't enough to affect fuel mileage or idle quality.

I do use Shell gasoline exclusively though.

2002 X5 3.0 242,000 miles
2004 325i 107,000 miles