My VP Free is about 40 pounds and I ride up Vedder regularly with it. While riding a porky bike up hill can be done by anyone, the question should rather be, do you want to do this regularly. I love the discipline of earning your downhill….I'm a masochistic, aerobic ex-xc racer type. I love it. The fact that there are but a small handfull of guys that will actually come out to ride with me when a shuttle isn't involved is testament enough to how popular this is. And I can say from the bikes I have tested over the years that there are definitely very different quality of rides when you are pedalling up. Some 40 lb bikes are easy on the rider (my VP Free is a case in point). Others that may even be a tad lighter will make you see visions of heaven before you get to your destination.
I am going to suggest that in order to max out your fun quotient, you should endeavour to keep the pork off….the wheels will give you the greatest bang for your buck and tubeless will definitely help. Excessive rotational pork is gonna rip your heart out. Good strong wheels, do not need to be Atom Lab heavy. You can save weight up front as well without loss of performance (have you seen the retardedly light 09 WC Boxxer?). Everything else that saves weight will help but not as much as in these 2 areas and it will cost you substantially.
Regardless, you need to get a star on your crown for making it your goal to ride up……rep coming your way brotha.