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
4 x 4GB DDR3-1600 Corsair Vengeance CMZ8GX3M2A1600C9B (16GB)
Graphics card(s)
MSI GeForce GTX770 Gaming OC 2GB
Sound Card
Realtek High Definition on board solution (ALC 898)
Monitor(s) Displays
ViewSonic VA1912w Widescreen
Screen Resolution
1440x900
Hard Drives
OCZ Agility 3 120GB SATA III x2 (RAID 0)
Samsung HD501LJ 500GB SATA II x2
Hitachi HDS721010CLA332 1TB SATA II
Iomega 1.5TB Ext USB 2.0
WD 2.0TB Ext USB 3.0
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