1. Xeon's are best suited for a multi processor workstation/server setup.. If you are building a single processor everyday use and/or gaming pc, going for a xeon setup will become costly.. Best to stick to desktop processors in that scenario..
2. There are newer video cards available.. Might want to check them out first..
3. There will be no real world performance improvements.. Its just that most current platform motherboards require DDR3 ram modules..