I'll post results (and maybe pics) when I've finished the upgrade.
Don’t go AGP please, you’re locking yourself into a dead tech that way. ATI or Nvidia’s mobo integrated video should be almost as fast as the card you have, certainly more than adequate if you aren’t gaming. Use newegg.com’s motherboard power search and check 1xPATA, some of the onboard video options, PCI-Express, and look through the results. Find one with some reviews and a good average rating.
Also, don’t listen to the people who are urging you to overbuy. It’s always better economic sense to buy tech 6 months to a year behind the bleeding edge, and upgrade more frequently (assuming you enjoy fiddling with your hardware and it’s not cutting into billable hours, but I’m making that assumption, because you are looking into building a machine instead of buying a ready made model).
It is hard to get 2 PATA channels on a motherboard that isn’t end of life or refurb though, so you might have to ditch the CDRW/DVD drives depending on how many HDDs you have. Better yet might be to buy a SATA HDD, move all your data over, then hook the CDRW/DVD up to the PATA channel and throw out the old PATA HDD before the sucker crashes.
A 450W power supply may be ok for the kind of system you’d build, but it’s probably 20 pin, and new motherboards may require 24 pin.
Also, if you aren’t used to linux already, you may want to start by installing the XP software from your old system and using wubi to run ubuntu linux from within XP. I use open office instead of MS office, and some people use MS office within WINE or crossover office. However, if you’ve already bought XP, why throw it out? It’s not like it’s obsolete.