I see this questions come up quite a bit about the interoperability of x86 and x64 domain controllers. Does replication work? Do the tools cross over well? Any gotchas that we should know about? Etc. Well I'm here to tell you that here at Microsoft we've been running a mix of 32 and 64 bit domain controllers since beta Wk23 SP1, and I can report back to you "Don't worry about interoperability of the domain controllers".
Replication works as expected.
Remote management tools connect the same.
When you TS everything feels the exact same.
You wont even know if its x64 or x86 unless you open up task manager and check out how much memory LSASS is using...
When I have engagements with customers, they ask if they should upgrade to x64 in their environment. My response is: that depends. Should you buy x64 compatible hardware? Yes, the cost difference is not that drastically different and this prepares you to install an x64 OS in the future. At the same time, an x86 OS installs and works as expected on the hardware, although you don't get all the nice benefits of running in true x64.
Do you need to spend tons of extra memory so that the DIT can be cached? That depends, and you can read this article for an explanation.