It works just fine on x64. You won't be able to run a boot time scan on x64 with it, but it's not a big deal. Pretty much the difference in the pro versus the free is you get scheduled scans, and one extra security module, a script blocker.
The Beta 5.- they've been working on is progressively getting better, the user interface is way better, it's being built into a more overall security suite. It's free as the beta, once it's finalized they will have a free version of it and a paid one just like the 4.- one is now. comparison between free and pro Beta 5 avast