The initial batch of DX10 games are likely not much, if at all, better than the upcoming DX9 games. This doesn't influence my decision at all. I usually buy a card and keep it as long as possible then get a new card with a good power boost. My current plans already have me upgraded to an X1900XT, this will tide me over for a good while. My next planned upgrade is an X2 3800+ when it hits the $160'ish price range after Conroe's release next month. Unless the Opteron 165's which are in a similar price range as the X2 3800+ drops to similar levels I'll buy that instead. I already have 2GB of RAM so that's set. Maybe another 400GB+ HD. Those TV shows and anime and uhh...stuff...fills up the HD fast.
I don't think I will upgrade my CPU and motherboard until DDR3 hits as I currently see no significant reason to move to AM2 or Conroe. The cost of replacing the motherboard, RAM and CPU to move up doesn't make sense considering the rather minor (at this time) performance gained. Virtualization sounds cool and it will be available in Conroe I believe as well as in Socket AM2 CPU's but I don't think, due to immature software, it will be ready for prime time for a while, much like the 64 bit extensions in the Athlon 64's.
Current plans for video card upgrade is the second generation of DX10 cards. Which likely will be revised R600 and G80 cards. Hopefully with better temps and power draw than we're currently seeing. Now, this can all change depending on how well The R600 or G80 wows me. The performance of these cards would really have to knock me over for me to jump.