I remember a time when I think you'd have jumped on something like X64
without a second thought :)  I'll make a short story longish for ya.

The problem I had with installing RoE was that someone made a tiny booboo in
the setup.ini file compiled into the .msi installer.  XP Pro x64 is seen as
MS server 2003 or somesuch and while x64 wasn't excluded in this, it wasn't
included either.  Someone unpacked it, made the correction then repacked it
and it worked.  simple enough.  Doom3 and RoE run great!!!

The company I work for is still using NT4 man :)  Sheesh... can't even use
USB ports.

There are a few *small* differences in the interface.  One visual.   On this
machine, most all of the 32 bit software runs and you don't see a difference
between the two systems.  When you get something that is native 64, then you
feel like you got something :)  Some *few* games have problems in x64, but
then it might be they're having problems with my dual core CPU and NOT x64.
I've seen a couple problems with dual core cpu's but not with x64 itself.
Most do just fine.  I dual boot.  Well, I *did* for the first couple of
days.  I've run x64 only for a month now.
Device drivers are the big issue.  But most of the popular brands have now
jumped on the 64 bit wagon with Vista looming closer every day.
CPU, chipset, video are cool.  Those were the first manufacturers to cough
up 64 bits quick.  All of my hardware now has 64 bit drivers.
Hewlett-Packard was the last, and these are beta 64 bit drivers and are a
'unified' package.
I did originally try out the 'trial' version of XP Pro X64 thru the
AMD-FarCry 64 program on the AMD website.  That was back in August.  It's
when I started this build, and yep, I originally planned to go thru an AGP
setup first, then just get a PCIe mobo and 7800GTX later.  So the plan
worked out.  I really liked the game performance in 64 bits a lot.  But
drivers were comin up short.  So I waited a bit on x64, and bought a new XP
Pro 64 bit edition OEM when I picked up my PCIe mobo and first 7800GTX.
Then with all 64 bit drivers for everything except the HP printer, I just
stayed in x64 full time.  The 32 bit version is still there if I ever need
it.  So far, I haven't at all.  The move over to x64 sounds all big and
hairy, but it's not.  PlanetAMD64 is a great resource for x64 users.

The stuff:  the following all have 64 bit drivers/software.  All this works
great!
AMD X2 4800+, Gigabyte mainboard nForce4 SLI chipset, nVidia vid cards, 6800
thru 7800GTX, Epson Perfection 4990 Photo flatbed scanner, HP Photosmart
7660 inkjet printer, Logitech Extreme 3D Pro joystick, Canon Powershot S2 IS
digital camera and software, various optical mice and PS2 keyboards, Wacom
Graphire3 6x8 pentablet, Samsung Syncmaster 930B's.

I have an older Hauppauge WinTV card, and I already know they do not have 64
bit drivers or software out yet for any of their stuff.  It's ok, I'll use
the P4 system in the living room if I want to use that card.

It's interesting that you have an A64 3700+, it's the same one I recommended
my son buy for his (my old) s939 AGP mobo and 6800GT AGP.  I left the 2 gigs
of ram in it for him.    The 3700+ is in the same stepping as the FX53,
4000+, X2 4800+, and has a 1m L2, venice core.  I think there's a sweet spot
there in performance per watt as well as per dollar.   I wanna know how it
runs Q4 and FEAR for ya!
HTH,
McG.