Radeon X1900 XTX is a high-end GPU based on the 90nm variant of the R500 architecture.
It's based on the R580 Core and offers 48 Pixel Shaders, 16 TMUs and 16 ROPs, on a 256-bit of standard GDDR3. The central unit runs at 650MHz and the memory clock operates at up to 775MHz.
Its therefore a further clocked Radeon X1900 XT with a twice as big frame buffer and should only perform slightly faster.
Most of today's games might be playable at medium settings although the low preset will have to suffice on some. As it's not based on a Shader-Unified architecture, both DirectX 10 & 11 games aren't supported.