dd-wrt is firmware like Tomato. Try the router out by itself, if it works and you are hapy enough with it you can leavi it alone. If you find the connections limiting than you can llokat dd-wrt or tomato and upgrade the router.
When I do this, will Internet go down, any issues with my network, will computers need to be setup again on the network, or what?
And
Index of /dd-wrtv2/downloads/v24-sp1/Consumer/Linksys/WRT54GL_1.1/
There are many items, what do I download?