i recommend the ATI rage fury specifically because i have one and
it works very nicely. (but you will have to install XFree-4.0.2 for it)
you need XF-4 to use the xgamma command anyway.
youll also need to go with ATI for hardware gamma. it wont work with
a matrox card (which is the only reason not to get one) and its support is
not mentioned for nvidia either (havent tried it)
if it means anything, i suspect the matrox card to actually be better, just
not at its linux support.
just make sure you dont get one based on the rage 128 pro, those are not supported.
the rage 128 based ones are.