The difference between 1.5 and 2.0 GHz is not as big as it looks. Upgrades are generally only worth it if you double the speed. I doubt you will notice much real world improvement.
If you are going to buy memory anyway, why not replace all 3 items - processor, ram, and motherboard? You can still get decent Socket 478 boards and 3.0 Ghz processors - and DOUBLE the FSB speed - THAT will make a HUGE difference, along with the upgrade from SDRAM to DDR.
http://www.newegg.com/Product/Produc...82E16813131515
-OR-
http://www.newegg.com/Product/Produc...82E16813131484
http://www.newegg.com/Product/Produc...82E16819116171
http://www.newegg.com/Product/Produc...82E16820145440
If $188 is too steep for a processor, give her a Celeron-D. It's still a 533 FSB, better than your current 400.
http://www.newegg.com/Product/Produc...82E16819112186
This will leave you a perfectly good 1.5 GHz motherboard, processor, and ram as a core for a Linux project or something.