Someone claims (https://groups.google.com/forum/#!topic/sci.crypt/JSSM6Nbfwe...) that it's just as easy to spoof 64 bit as 32 bit keys. I have no idea whether to believe him, but even if he's full of it, it's prudent to assume that if the 32 bit attack was practical 20 years ago, 64 bits is within reach today.
That's a post from 1996. I'm not sure how "easy" was calculated. Possibly both were hard but tractable, but 64 bits should have been much harder.
Today it is significantly easier to collide a 32-bit key than a 64-bit one, but both are pretty easy. A 32-bit key can be collided in 4 seconds on a GPU, according to https://evil32.com. It can certainly be done on normal desktop hardware in hours. For a 64-bit key, 'JoshTriplett calculated (last week) that a collision would take 15 days if someone built hashing hardware of comparable quality to a commercial Bitcoin miner:
(Incidentally, I am very pleased with Bitcoin having created a liquid market between cryptographic computational speed and money, so we can answer these sorts of questions precisely.)
