All drivers can be modified. It's just a matter of dedicating the little free time to what is really worth or makes sense.
Video card generations support a certain version of DirectX. Because Windows video handling is so deeply based on DirectX, this means video cards are somewhat tied to specific Windows versions.
The X series are DirectX 9 cards, this means Windows XP is their OS.
The HD 2000-3000-4000 series are DirectX 10 cards, so they'd be good for Vista, but Vista was a failed OS, so support was granted for 7. However, this support was never good.
The first generation with full Windows 7 support (DirectX 11) is the HD 5000.
Between XP and Vista there was a huge architectural change in video drivers from which consequences we're just starting to recover.
So probably there's a compatibility aimed driver (9.3) for 7 (Vista actually that works in 7 too) for the X600 that could be modded, but when we did the transition to 7 it seemed like a good idea to draw the line starting with the HD series only.