Harold has just started a great thread which I'm sure will get a load of comments. From my perspective I would buy x64 today (in fact I have done at home) however the full software stack isn't always available - yet! I think it's inevitable that x64 (64bit) will dominate in the next few years and I expect (as Intel and AMD have been saying) that most system shipments this year will be x64, but due to the issue of lack of support in the software stack many will still run x86 Operating Systems and Applications. The good news is that the x64 chips will be out there for us to take advantage of...it's just a matter of time.
Harold said he would like to "see the OS and Applications go 64 bit and leave the 32 bit world behind" I agree with this and we are starting to see the migration mainly in bespoke workloads but given the x64 advantages and availability that will only increase (and quickly). I have a feeling many people are waiting for the dual and quad core chips (it's similar to those who wait for a SP1 from Microsoft) before dipping their toes in the water.
I'd like to ask about your experiences with 64bit. I know I'm a fan as I've seen SQL server fly on large Itanium systems, and there is a secondary advantage (at least in my experience) and that is simplicity. I should make that clear - when I mentioned "simplicity" I mean that 64bit can give you clarity and I should give an example to explain. Last year we took a 32bit SQL database on a 32bit server (being tested in a Microsoft lab with a customer), detached the database and attached it to a SQL 64bit machine - it's that easy believe me (using a shared SAN between the two servers). Why did that gives us clarity or simplicity, well on the 32bit system the Perfmon and Task Manager output was chaotic CPU and Memory spikes galore and trying to spot the bottlenecks difficult. The Itanium system (and x64 should be the same) showed virtually a flat line for CPU and Memory but disk was bottlenecking like crazy - a few extra HBA's solved the issue. Basically the system was having a problem loading data into memory quick enough - hence the disk and memory activity, given the flat address space of 64bit the issues went away. As a note the extra HBA's when applied to the 32bit system didn't make much difference.
My parting question would be - who do you think will win the dual core battle? The architectures are very different and one looks better to me, I guess benchmarks will help us to decide. The chip architectures can be seen in a simplistic form on the slide "Systems architecture make a difference" in this deck in case you're interested.