64F is on the low end, but should be perfectly fine for your beer. What type of yeast did you use? If nothing else, it may just take a little longer to completely ferment out, but it should result in a great beer. The higher you go up in temperature, the more yeast "off-flavors" develop in your finished beer. Keeping it in the lower 60's as you are right now should work out perfectly for you. Remember, RDWHAHB!
"Brewers enjoy working to make beer as much as drinking beer instead of working."