You have not read enough of Houdini's online user manual. First look at the 2.1 Installing Houdini page.
Specifically, it says you'll get 30% faster performance with 64-bit over 32-bit. It says it's optimized to use up to 6 cores. And Intel i5 will run it faster than an equivalent speed AMD Opteron. Their home page says it will use up to 4 GB of RAM for hash memory. Obviously from that, you'll want to get the highest clocked Intel i7 you can find with a 4th generation CPU.
But if you get Houdini 3 Pro instead of Houdini 3 Standard, it supports up to 32 cores and 256 GB of hash memory and NUMA-architecture (whatever that is). Their example performance comparison shows on a 16-core dual AMD Opteron-6128 box that Pro has a 20% speed gain over Standard. With the Pro version, you'll want as many cores as you can get, because that's the number of threads you'll be able to run at once. Intel i7 will likely be better than AMD for the Pro version as well.
Definitely read everything in the online Houdini 3 User's Manual, especially section 3 about its configuration. That tells you how to optimize it.