Processor wise you'll gain about 5 - 10% performance, but only in x64 Vista Windows compiled applications.
Any 32 bit processes may run on top of a WOW64 (Windows on Windows 64) layer, and thus perform about the same in a 64-bit Windows Operating System as they do in a 32-bit one.
In Task Manager (Processes Tab) they usually have a * in their process name to indicate they are 'older' applications.
Also support for 16-bit applications (including some installation software) is expected to be unavailable in Windows Vista, as it is unavailable in Windows XP x64 Edition. (This is a damn good thing, and the OS can substitute some installers with newer versions on the fly
)
However Half-Life 2 (Single Player and Lost Coast only), Far Cry, and other games and software are available in x64 versions and/or have x64 patches available when the required operating system and hardware is detected.
Memory and OS Disk Cache performance wise it is harder to say, but with 2 GB of RAM expect better performance than you have now on the new platform.
The Conroe will be x86/x64 supportive from the ground up, and includes far more enhancements than most people here (eg: AMD Fanboys) are willing to admit.
UPDATE: The Intel x64 processors, nor the AMD ones either, use emulation for x64, they both support it natively in the hardware. Just the Pentium 4 600 seres (and some 500 series ?) implementation was not a 'clean' one, so performance in x64 only rises 5-10%. The K8 (Athlon 64 / Opteron) by comparison was designed for x64 from the word go and scales quite nicely (Over +25%, vs Win32 apps, in some software) in x64 applications.
The differences are substantial enough that compilers offer to generate P4 EM64T optimized code, or AMD64 optimized code, or a balance. Ideally each optimized code path is generated, then the CPU type is detected at execution, and finally the more ideal path taken. (Although this requires much more coding effort, but it is ultimately worth it).
(Windows XP x64 Edition was made using Opterons, back when they clocked around 1.6 GHz, because nothing else was available, something AMD should be proud of but doesn't advertise very much at all).
PS: I ain't a brand fan either, I'm an architecture fan, even though currently using Opteron 270's still think Intel IA-64 on 65nm would be far nicer
CPU architecture.... just so long as the software was released for it.