I recently did a LOT of research in the field of 64 bit and MythTV. All of the experts agree that running Myth in 64 bit is (as of now) a BAD idea. The reasoning is this... 64 bit doesn't give you faster performance, it gives you more fine grained performance. If you use sound processing as an analogy it becomes easier to understand.
If you pump your raw sound (a live recording for example) into a 32 bit system, you can then adjust 32 bits of data. You have 32 degrees of variation. If you push the same raw sound into a 64 bit system, you have 64 degrees of variation. This means you can fine tune the sound better. BUT - you won't necessarily be processing it faster.
Since all of the content delivered through Myth, even HD content is easily handled by a 32bit pathway, there is no reason to go to 64bit for Myth. 32 bits is more than enough of a pipe for the data (Even Blue Ray and HD-DVD, I checked). There's just not enough data there to fill 64 bits.
As a matter of fact, on the MythTV mailing list, some users tried compiling myth for 64 bit from the source code and got horrible results. Others tried running regular Myth on a 64 bit OS and found that the results were also horrible (90% CPU use vs 12% CPU use for transcoding) due to the fact that the myth components were running in 32 bit emulation.
With all that in mind, it is HIGHLY recommended you have at least 3.6GB P4 class processor and 2 GB of RAM to do HD. (debate rages on best mobo manufacturer. I stick with MSI high-end gaming boards) I advise anyone considering HD to go with a dual core CPU. Right now the AMD FX 64 is king, despite it's lower clock speed, benchmarks show it WHOMPING on every other CPU when it's doing video or audio of any kind, transcoding is mind-bogglingly (is that a word?) fast. It even beats AMD's dual core 4400.
If you do go with a dual core system for your MythTV, be sure you are installing a 32 bit 2.6 SMP kernel. After that, install Myth and everything else takes care of itself. Some day we will go 64 bit, but it's going to take some work, and the results won't be an improvement, just the status quo.
More on this can be read here -> http://www.linuxis.us/linux/media/howto/linux-htpc/