> will a X1950GT work?

While only Pro and XT versions are listed on the official hardware support page, also GT and XTX versions should work, as they use the same GPUs (RV570 on GT/Pro, R580+ on XT/XTX).

> are either any good in Debian Linux?

GeForce 6xxx and Radeon X1xxx should work to a similar degree on Power(PC) Linux. Both share the restriction to OpenGL v2, which may be too old for more recent 3D software requiring OpenGL v3 (OpenGL v4 doesn't work on big-endian Power(PC) Linux anyway).
There's a useful list linked to from another thread which, while being hosted on the Void Linux Power(PC) site, shows the general state of GPU support in Power(PC) Linux.

> K2000 Quadro (for Linux)

No chance on big-endian Power(PC) Linux.