here r the specs for both the x800 pro(8*agp) and the x600 xt(pci-e)....
x800-
Specifications:
Chipset/Core Speed: ATI RADEON X800 PRO/475MHz
Memory/Effective Speed: 256MB GDDR3/900MHz
BUS: AGP 4X/8X
Ports: VGA Out(15 Pin D-Sub) + TV-Out(S-Video/Composite Out) + DVI
Pixel Pipelines 12
Support 3D API: DirectX 9.0, OpenGL 2.0
Cable/Accessories: 2 Adapters, 4 Cables, 1 CD, Manual
Max Resolution@32bit Color: 2048X1536@85Hz
x600-
Specifications:
Chipset/Core Speed: ATI RADEON X600 XT/503MHz
Memory/Effective Speed: 128MB DDR/736MHz
BUS: 16 lanes PCI Express
Ports: VGA Out(15 Pin D-Sub) + TV-Out (S-Video/Composite) + DVI-I
Support 3D API: DirectX 9, OpenGL 1.5
Max Resolution@32bit Color: 2048X1536@85Hz
Cable/Accessories: DVI to VGA adaptor, TV-Out adapter,ASUS Driver and Utilities, Software Bundled
it looks like the x600 has a faster core but the x800 has faster memory. im goin with an lga 775 board, so the x600 is my best choice for graphics card and it dosnt look like there is much, if any performance loss versus the x800.
edit: the x600 only comes with 128 mb ram :/