I gained 40 pds w/ my 1st pregnancy..but w/ my 2nd, I was so concious of what I was eating that I didn't gain any extra weight!
Remember, that eating for 2 is just an old wives tale! Just make sure you really eat healthy meals! You'd be surprised how important that is to the little one

also, walking is a GREAT excersize!! I know there are some fitness dvds that focus on pregnant woman, you might wanna check that out.
I'm with Mal though, Consult your doctor 1st!!
btw, CONGRATS!!! babies are soo precious!