[1lacca]

Intel is very good in compiler optimization, and these CPUs gut a bunch of 'hidden' registers that are available both in 32 and 64 bits mode. They can really speed up some decoding tasks, since they are obviously even faster than cache.
Also, in such encoding things extensions like SSE and others can make a difference as well, if the code is compiled to take advantage of them (the added registers and special vector instructions, etc.).
When you use dual/quad cores, or multiple CPUs it is also interesting if the application itself takes advantage of multiple cores. Some scales very well on such systems, other see no performance gain at all.

[NickDMax]

Ok, I have noticed one problem with WinXp x64 -- 32bit and 64bit programs don't "talk" -- so for example I installed the 64bit version of python, but found that 32bit programs that use python can't call the 64bit version. You have to either only install in 32bit version or install both and make sure that the other program knows where to find it.

I imagine that Apache is going to give me the same trouble with perl and PHP if I don't compile it for the 64bit platform.

[SpeeDemon]

I'll start by saying that I have never run XP64, but I have run both Vista 64 and 32 variants on the same hardware. For most of what I do, vista32 is the best option. Most video codecs and drivers are still optimised for 32 bit. I have come across the odd application here and there that fails to run at all in 64 bit.

However, I will definately agree that Vista 64 is lightning. The general "peppy" feeling that vista gives over XP is even better with 64bit. Unfortunately the version of Vista my employer supplied me with is 32bit. I would give up some application support for those extra pipelines.

Intel Core 2 Duo/Quad CPU's have a large amount of optimisations in them to improve on caching, power consumption etc. 64bit processors can work faster in a virtual memory environment than they can with physical memory, proving their efficiency.

These CPU's also have larger cache widths. The Core 2 is currently at 256bit, not 64. I don't think there would be much increase in overhead with that.

Anyway, CPU's are far more complex than my brain can comprehend. But why did you opt for XP over Vista? You crazy, man.

[NickDMax]

in hindsight I am starting to regret the XP decision. However, the decision was made because I have just heard so many bad things about Vista... and it pisses me off (the different versions), and its expensive.

it is actually more the price that anything that made up my mind. I had already reached my budget for my computer when occurred to me that I would have a 64bit processor with a 32 bit operating system (since I was just going to install XP from the old computer). So I splurged and got myself a 64bit OS... XP was the cheapest (by a long shot).