Please explain what you mean?
Longhorns 'different graphics system' is called "Avalon"... It's nothing particularly spectacular, Avalon is just a nice way of saying DirectX10.
I dont know where you heard that pixels are going to be replaced but i can confirm that, that is NOT true. But Avalon DOES provide lots more support for monitors with a high DPI's and greater system stability at higher resolutions.
This Avalon interface will allow more detail to be put into FS2006 and may help with FPS.
Also avalon will not only be released in longhorn, but will also be released as an upgrade to WinXP and Windows server 2003.
Avalon also boasts more features, but i wont go into any detail... You can download Avalon now from the Microsoft wbesite, just click here.
Hope that helps... If you need any more info just post...