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 $US100 or less. Ranging from $US100 down to $US55, 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.