Direct X 10 will come out right before Vista comes out (supposedly December of this year)
However not all of todays cards will be DX10 compliant, I believe anything with 256megs of 256bit video memory running DX 9 will qualify for Direct X 10.
I can guarentee you that your X1900 will be able to handle direct x 10. However your GeForece mx4000 wont.
I honestly really wouldnt put off building a rig to wait for DX10 to be offically released because vista is already on its 2nd beta and I know for a fact the intel will be shipping Vista as soon as September of this year...
How do I know this (because I know everyone is going to whine and mone that I'm just pulling it out of my butt) If any of you are MCP (microsoft cert. pro's.) there is a required training evernt here in PDX (any every other state) in september to qualify for Vista cert's.
Now I dont know this for sure, but I'm thinking most of the GPU companies have come out with DX10 software already. If you look around both ATI and Nvidia have already released Vsta drivers and patches on top of those, so why wouldnt they be working on the software for their 500.00 cards?