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!