A hundred bucks isn't what it used to be. It won't get you a new pair of Jordans or take you halfway to buying your way out of a Union army conscription. But it'll buy you a half-decent video card.
AMD has released three new cards for $100 or less. Ranging from $100 down to $55, the Radeon HD 6670, 6570, and 6450 are decent upgrade options if you're still chugging along with an older or integrated chip, trying to peel off headshots at 10fps on minimum settings. However, they're not without their drawbacks-the fan noise is apparently pretty noticeable and there are slightly better options for just a few dollars more.
That said, if you're looking at these cards in the first place, there's a good chance that those couple bucks matter to you, and these are more than enough to get you up and running on today's newer releases: the GDDR5 6670 (the $100 model) managed a perfectly respectable 45 FPS in Call of Duty: Black Ops. So if you're in the Ramen, water and Wonderbread crowd, it might be about time to upgrade that graphics card.