Now I have experience with some nvidia GPUs, AMD-ATI GPUs and VIA GPUs, and and old... was it S3? in my 486. And from these years I can tell that both ATI and Nvidia did a fair job. Both on Windows and Linux. It was never perfect, my Geforce2 and 3 ones are way unsupported by now, I don't blame nvidia that much, it would've been nice but alas they discontinued support. So basically - if there wasn't the xf86-video-nv they would be bricked. That is nasty considering what horrible amount of money I spend back in the days for the Geforce 3.
I switched it to a passively cooled Gef2 (nv11?) in the old box since the Gef3 sucked too much power anyway. And since I nearly only do low level GPU usage stuff with that box it is ok.
The driver itself... I only used nvidias closed driver for a short time in linux (was fairly ok) and on windows. From the windows side there were some issues that were fully broken and some that just worked.
Today I like to have my ATI's around for 3d in Windows (gaming) and under linux I prefer to stick with xf86-video-ati after some time of xf86-video-radeonhd. I threw out fglrx at the beginning of this year when I switched mainboard. It needed a 2.6.29-something at least for the chipset but fglrx was stuck til summer 2009 with 2.6.28. So I just keep waiting for 3d and all other kinds of goodness to happen in the free driver structure (all chips are R600). But it is fairly okay for me. And: There is no "legacy" issue. Even if I had older cards there would be a free driver offering all kinds of things, thanks to spec releases.
And the VIA... I don't want to rant again. But the drivers aren't really doing much besides some 2d (you're happy when at least MPEG2 runs). These chips don't eat much power but they also don't bring you much pleasure.
So with nvidia you might be better off with a recent card and not the older ones, and the guarantee that these cards will brick someday and you'll prolly never see any specs.
From the interview nvidia <-> Phoronix they won't harm noveau but then... well... I actually waited for noveau to happen since my later Geforce 3 times. I don't wait anymore. I use ATI. Fine if they'll ever get noveau running but then I doubt they'll have support for these old nv11 nv20 chips.
With AMD-ATI you'll have a choice in drivers and the older ones should do fine with the free drivers while the recent model will require some waiting or having to arrange with fglrx's likings to Kernel and X versions.
Stop TCPA, stupid software patents and corrupt politicians!