Should surgery be done?

I have a female labrador, who is six year old and was nutered when she was 1 year old.

she is very overweight and weighs 33.8 kg. i cannot get weight off her.

today 6 th of april 2011, when running after a ball she injured one of her hind legs, i immediatly brought her to my vet who said she had damaged her cruicate ligament in her hind leg. he gave her an injection and sent me home with a bad of tablets - she is to get 1 a day for 1 week and then return to vet, whereupon he said he may have to perform surgery which he says will costs 400 euro (i live in ireland).

all day i have been reading information regarding surgery, and from what i can gather no one surgergy is successfull, the dogs weight is seriously against her, and some websites even say that studies have shown that surgery is often no more successfull than just rest etc.

therefore my question is should i go ahead and pay for the surgery (obviously money is a huge factor). i would have no problem paying for the surgery if i knew there was a high success rate, as i want to do the best for the dog. however im worried the vet will just press me to do surgery even though it may end up to be a total waste of money.

as an independant party what would you advise?

and if advising surgery - which surgery would you advise?

please take into account that i have been trying very hard over the last 2 years to get weight off the dog and she hasnt lost a pound. ive even been swimming her.