I doubt a Celeron 2.6GHz would perform any better than your sucky old 1.9GHz Williamette. Go to the CPU section at Anandtech and read the "clash of the 'rons" article, you'll see that the fastest Celeron they tested was slower than the slowest P4 they tested.
You could run a P4 2.6A (A meaning 400 bus) if you could find one, or the more common 2.4A. Some sites have been offering these at closeout prices due to their age, see if Tigerdirect can beat Newegg's price on these.
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