This just might work. The main problem is cost. I did find the part (661-5480) for $1299 here: http://www.macpartsonline.com/661-5480-l...
Of course you'd have to buy new RAM also. They're both A1286 machines.
However consider this, Everymac shows your current machine with a street value of $1200 - $1300.
Add to that the cost of the new board 1299 +1200 = $2499 + RAM + shipping + labor + no warranty
Cost of a new i7 from Apple (with warranty) $2199
Looks like your much better off selling your old machine and buying a new one.