You believe? You better be SURE or you'll brick the router! It has to match the compatibility list for make, model, and revision.
You have to follow the directions at dd-wrt for installation (and no, it's not plug and play). You're basically going to reset the router to defaults, connect to the router using a wired connection over a LAN port, and install the firmware update. The firmware update can be a bit tricky depending on the make/model of router. Once dd-wrt is installed, you then configure the router as a wireless ethernet bridge (aka, "client bridge" under wireless setup).
So first things first, locate your Linksys router (make, model, revision) using the
dd-wrt database. Select your revision and follow the link. You’ll see “Additional Information “ and a link to the wiki for that dd-wrt install. Follow the directions. If it seems a bit overwhelming, or you just get lost (a lot is assumed about your networking knowledge), come back here and ask questions (and provide the revision # so we know which wiki is applicable).
If you only need a client bridge, it’s possible tomato is also supported and might suffice (some ppl find it easier to install and configure).