The older British cycling manuals used to suggest a 65-in gear for freewheeling bikes; the medium gear ratio time trial folks used to set an upper limit of 72 fixed gear inches. I've been running right around 70 fixed inches for 24 years over the rolling terrain of what the realtors have taken to calling South Carolina's Lakelands with a fair bit of success, including club rides with roadies - but NOT hammerfest rides. As mentioned earlier, one attacks on the climbs, learns to love to dance on the pedals as the momentum slows, and to perfect spinning as smoothly as possible on descents. It helps if you ride with people who are familiar with fixed gears and understand they will need to pass you on descents.

I am in bensanford's camp - I vastly prefer fixed to single speed for climbing, for the same reasons he outlines.

Not mentioned so far, but be sure the frame geometry works for you on the bike to get the fullest use of all of your muscle groups. I like dropped bars because they provide different positions that help with getting into a spinning position, sitting way back on the saddle whilst gripping the bars near the stem and "turning 'em 'round" a la 1950s British club cycling doctrine, grabbing the brake lever hoods to stand and honk up the steep bits, and going down to the drops to muscle up short sharp uphill moments. I run 42x16 and 45x17 on the two bikes with 28 mm tires and 44x17 on the bike with 35 mm rubber.