Very, very sorry to hear that about your furbaby. Always sad when they are sick.
As Amy mentioned above, our son's dog, Doc, (seems more like DW, Marie's, and my dog as Vince is gone so much and Doc is here at home with us), was diagnosed with lymphoma about 2 1/2 years ago, beginning of '04. Doc was just over 4 at the time. We brought him to nearby Veterinary Specialty Center
http://www.vetspecialty.com/ and were told without treatment Doc would last maybe 4 to 6 weeks and be gone, virtually 100% fatal. With chemo, (quite expensive), most likely Doc would go into remission, for maybe 1 or so years. It would come back. If another course, maybe half the length of time in remission as the first. And then that is about it.
Vince decided to give Doc the treatments. If not, I am sure Marie and I would have. Doc went into remission after his first chemo treatment, but needed maybe 6 or so more for a complete course. He was a bit out of sorts after each treatment, just like a person, but after a couple days, was fine. He was in remission for over 2 years.
In about march of this year, he came out of remission. Started again on a chemo course. Into remission after the first. He has a few more to go and then will be done with this course.
I will say that for the approximately 2 1/2 years, though we know where it will end someday, Doc has had a wonderful life, enjoying it to the fullest.
I do hope your guy, whichever you decide is the way to go, enjoys his days too, be they many or few.
Here is a link to a thread I put up a couple months back, showing how well Doc is doing.........
http://www.disboards.com/showthread.php?t=1086980