1. When is DX10 suppose to be coming out ??
When Windows Vista comes out. DX10 will not be supported in Windows XP.
2. Would a 7900GT card not be able to play DX10 stuff, with a bios/driver update, surely it must.
Any current DX9 card should be able to play a DX10 game because those games will also be compatible with DX9. It will be a few years before DX9 support is dropped. Support for DX8.1 has only recently started to drop. Oblivion is a prime example, you must have a DX9 card to play (but there are hacks to get a DX8.1 card to play Oblivion).
But other recent games still supports DX7 cards. Star Wars Empire At War is an example. I loaded the demo onto my IBM T40 laptop with an integrated Radeon 7500 IGP just for the hell of it. It is definitely playable, but looks better on a DX9 GPU (naturally).
Support for DX9 cards will begin to die until DX11 becomes available or is announced. Therefore, a 7900GT or X1900XT will last you a few years, or until you decide they are too slow for your "needs".
Getting new hardware is great, and sometimes it's not. nVidia GeForce FX series was the first DX9 cards released. However, they turned out to be really, really bad at DX9 games. Basically the GeForce FX series was the worst product ever inflicted onto gamers. I think the Radeon 9700 had some intial problems too, but that could have just been rumors.
3. When should i update my rig, not bothered if its 2,6 or 9 months ?
Update your rig whenever you think the performance is no longer good enough for you. Upgrade to either the AM2 Athlon or Conroe. Conroe performs better, but if you
hate Intel then go for Athlon. The longer you wait the more likely a more powerful CPU model will come out. If you can wait until Q3 2007, then maybe by then the Athlon K8L desktop CPU will come out. The 45nm Conroe CPUs should be out by Q3 2007, until Intel runs into problems.