Republicans will be anywhere from 217 to 251, but the likelihood of them actually losing the House with 217 is highly unlikely. We're living in an age with a Republican congress and a White House that goes back and forth every eight years with an occasional exception of Bush's second term. I like it.
Having this...
White House: Republican
US House: Republican
US Senate: Republican
or this...
White House: Democrat
US House: Democrat
US Senate: Democrat
.... is the most dangerous thing that can ever happen in elections. In 2008, when democrats had complete control we got the Affordable Care Act (Obamacare). When Bush fist became president I was still a small child so I don't really know what Bush and his fellow republicans pushed through for one year (until the republican from Vermont switched to an Independent who caucused with the democrats) with their majority, but it was still dangerous to have one party control.
All political parties are lucky to have at least one chamber of congress or the presidency. America is lucky we aren't dominated by just democrats or just republicans right now. And furthermore America is lucky to have a a republican house for the next decade no matter how disorganized they are. It is completely dangerous for America to have complete one party control (especially for more than 2 years).
Nothing, or not that much. The line from Fahrenheit 911 went, "He couldn't get his tax cuts passed, he couldn't get his judges through and he lost Republican control of the Seante (six months not a year in), so we went on vacation and caused the terrorist attack."
Okay maybe not the last bit, but that was basically his underlying theme.
Even before Jefford's switch, it was a 50-50 chamber and Lott had made a deal to split control with Daschle. The Republicans really only got complete control for four years between 2003 and 2007 and during that period the only thing they did domestically was cut taxes, pass NCLB, pass the Medicare Part D before the election, confirm two judges in 2005, and change the bankruptcy laws in 2005.
That is Bush's domestic legacy, aside from that first tax cut, Sarbox and the Patriot Act.