I believe Direct X 10 will replace Direct X 9 upon Windows Longhorn release.
The x850XT is the fastest card on the market at the moment.
ATi has already unveiled their 512mb cards to the public. They will not have any special names, they will simply follow suit along with ATi's current naming scheme.
Expect the next generation of nVidia cards around the same time as ATi's, which will be in May.
Also, SLI is not compatible with every game so speaking from a broad and general point of view the x850XT is more versatile than two nVidia cards in SLI.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w