Hello,
The Beta drivers do run smoothly but not all the time. They will crash if you use them for gaming.
With the market the way it is you can get yourself a 9800GX2 or a 8800GTX card which can perform jsut as well as your SLi setup.
But DX10 is not fully worth it yet as there are few games that run it. Those that do can also run DX9 as well. DX10 will not be sufficent for at least another year and even then who knows since they have already announced DX11.
For the system you have there will be no benefit to jump to Vista. If you have XP running fast and smooth you will probably be upset with the way Vista performs and regret the decision. Vista runs good on 2GB but with 64 Bit you really should run more.
Cheers,
Mak