Network help

Ive recently upgraded my router from a 54mbps D-link to a WNDR3300 netgear 270mbps wireless router and i got the N wireless adapter for my pc which is working well apart from every time i turn my pc on it isnt automatically connected i have to unplug then replug in the adapter for it to find the connection.

my problem is Im using my xbox 360 wireless too on the same network but is using the 2.4 ghz instead of the 5ghz the pc is using which is due to the adapter. Im currently finding my conenction is less than half as good as it was on the old router. Is there any reason why this is. im thinking about using an ethernet cable from my pc to the xbox and net sharing but dont always want my pc when im playing.

can anybody give me any advice for this as im pretty new to networking.

thanks in advance

 

anyone?