Is it worth me getting...

one of these Geforce 7800 AGP cards?

With directx 10 and vista just round the corner, how long will it be before Game developers stop making games that support directx 9?

Vista has been available for the past 14 months and DX10 for the same period. In fact, SP1 for Vista has also been released a few weeks ago. There will come a time when game makers stop making games for older versions of DirectX, but this is no different to what it has always been. If you do want to upgrade to Vista, I would advise you to go for a new system that supports the PCI Express standard for graphics as opposed to the older AGP standard. At the moment, I am using a GeForce 7600GT PCI Express graphics card and, although it doesn't support DX10, it is giving me good enough performance for what I use my machine for. A graphics card update, however, might be on the cards at a later date.
Dwarf

DX10 is already out, in some fashion. If I look at my DXDIAG, it tells me my version is 10...though I know for a fact that some of the files are still 9x.c

Vista has both versions. This is why crysis will run in DX9 or 10 mode.