I personally helped my g-ma select her first computer since her LCIII, and we came to the conclusion to get a laptop, and the TiBook400 had just come out, she got one. She hasn't complained since. It may be a little slow and not much of a gaming system, but she says it works great for her. I say hold onto it (it is a pretty sturdy machine) and wait for the next round of laptop upgrades (I am betting on the iBooks being upgraded and same with the 15" PowerBook in the comming months). Unless you need a new system right now, I say wait and see. The DDR support would be worth it, and plus, the benchmarks I have seen on the 867 make it worse than the iBook in some respects. Wait for the 15" upgrade and maybe Apple will have fixed the lack of DDR utilization, and at the very least, you will be getting a better machine overall. After the announcement, I would listen to hear how long the wait will be for shipping, and as long as it isn't nearly as bad as the current 17" book, well, I would probably go for it. And if all else, you could get a previous generation PowerBook for a little less than you would spend now.
Just my two cents worth