I've seen cards on both sides (ATI and nVidia) under $100 in Fry's "in-store-only" ads lately.
If you look
HERE you can get some of their online prices.
They've got a BUNCH of 128 meg, 8x AGP cards that are under $100.
If you bump to $110 they've even got a 256Meg BFG card running a GF FX5200.
I usually buy BFG cards for my gaming rigs because I've had good luck with them for years now, but I've seen cards from other manufacturers that perform equally as well.
The CARD end is important, but the TV end is equally important.
I've got a Radeon 7000 card in my cabinet, that puts a bad picture to my 20" no-name TV, but a relatively good one to my Toshiba widescreen.
The BFG GF4 card I have puts a slightly better picture (still pretty bad though) to the 20", but about the same to the widescreen.