I think for that 70 for the XP, it isnt worth it. I have an 1700+ XP and my father has a 2800+ or something, and its not a whole lot faster. However, my 3000+ 64 feels much faster with same RAM amount and HDD speed (7200rpm w/8mb cache). Maybe get a used one? That ASUS is still a good board.
Keep your eyes peeled at outpost.com if you don't have a Fry's. There are really good combos, basically a free motherboard with the purchase of a processor.
True dual core isnt much use right now, but it will be. If you ever plan on gaming in the near future, many games are being written to take advantage of it. Plan on watching HD movies? Minimum dual core AMD X2 4200+ or Pentium 830 or higher. Thats with software encoding too. Without it, itll max out both cores. How about video/audio? My 2.4ghz takes 14 minutes to compress a 7.2gb movie to fit on a 4.4gb disc. My brothers dual core at same frequency, same cache, etc... takes 8 minutes.
Try running diskeeper, antivirus scan, listening to music and surfing with firefox with a single core. Itll stutter. With a dual core system, hardly.
If you can, get it. Not that much more and you wont be kicking yourself later.