Well first off:
Unless there is more than a $200-300 dollar difference in the price or too much to afford, ALWAYS go for the A64 over the XP-M. The AMD Athlon 64-M runs at LEAST 20-30% faster in every benchmark I have seen. And is ready for the future
Personally I have the A64 3000+ on my Emachines M6805, and it blows away my 2200+ Athlon XP desktop antique by at least 200%.
Unless there is some other overwhelming consideration such as price or configuration ( for example the let's say the XP-M has 1 GB of memory and a Radeon Mobility 9800 with a 17" wide-screen with a dual-layer DVD burner..., and the AMD 64 has only
128 MB of PC133 memory, a 10.4" LCD screen, no CD/DVD drive, a 10 GB HD, a GeForce 256 8MB video AND is made by Packard Bell/Commodore AND costs $1000 more....THEN get the Athlon XP-M!
Seriously, give us more specs and I bet the A64 wins the configuration fight...
Answer Part 2:
The only time you would be running in 64-bit mode is when running a 64-bit OS, i.e. Windows XP 64-Bit Edition (BETA). In WIn XP, it is ALWAYS in 32-bit mode.
Where the difference lies in Windows 32-bit-world...is the 1 MB L2 cache; the HyperTransport vs. FSB; the SOI CPU with 105 million + transistors vs. XP-M's 50? million?...and the AMD 64-M 3000+ only cost $30 more part-for-part.
Answer Part 3:
In Windows XP 64, you still don't get a choice of 32-bit vs. 64-bit processing. It's all dictated on the 'fly' by the particular software you are loading. In that case, a 32-bit program will USUALLY run the same as it would in a 32-bit Win XP environment...with a few exceptions. Drivers in particular almost HAVE to be 64-bit before they will even be installed by Win64.
Win XP 64 Beta Build 1270 runs great except for a lack of drivers for the ATI Radeon Mobility 9600, and few if any wireless drivers. I can't benchmark it until I get a true video driver but it seems very reponsive, as would any freshly-installed and pretty-bare OS LOL!
Good luck in deciding, hope that all helps. For next time, maybe give more specifics and we can get more detailed comparisons for the two systems. if I can type anymore after this
eMachines m6805 great specs for $999!
Mobile Athlon 64 3000+ | 512MB PC2700 | 60GB 5400RPM | ATI Mobility Radeon 9600 @1280x800 | 24x CD-RW | 15.4" widescreen | XP SP2