A 64-bit chip is like a 32-bit chip which can do more. Anything a 32-bit chip can run, the 64-bit chip can run just as well - But a 64-bit program won't work on a 32-bit chip because it's trying to use features that aren't there.
The main "limitations" of the 64-bit chip are rooted in the operating system you run on it. For example, the 64-bit version of Windows doesn't come with the NTVDM required to run DOS programs (though there's nothing stopping you from using DOSBox instead). But there's no need to emulate a 32-bit chip because all the support is already
there.
The system does change "mode" depending on what it needs to do, though. For example, to use 64-bit features it enters "
long mode".
Anyway...
Windows XP Professional (5.1, Build 2600) Service Pack 2
MS-6380
AMD Athlon(tm) XP 1800+, MMX, 3DNow, ~1.5GHz
512MB RAM
DirectX 9.0c (4.09.0000.0904)
HIGHTECH EXCALIBUR RADEON 9550SE Series
SB Live! Audio
Runs 2D as well as you'd expect, given that the client also worked quite well with something like 32mb RAM and a 100-200mhz CPU once upon a time... Probably still does but I can't be bothered to check.
KR seems to work quite well also, though I reckon a bit more RAM and a faster CPU would help smooth out the bumps just a little bit. It can take quite some time to draw the screen after warping somewhere entirely different to my current locale, and it doesn't like having too much going on at once in my area, but when just running around there's nothing to complain about.