Vista 32-bit to 64-bit upgrade and notebook hardware requirements

I used to think I knew the hardware spec for 64-bit Vista...but now...it
seems Microsoft has either hidden or hedged the requirements. So let's see
what is said here...

It was my understanding that for Vista 64-bit to have a chance of running
decently, you had to have a dual-core 64-bit processor with 2GB of RAM and a
video card with at least 128MB of dedicated RAM. Is this still true?

For example, what are the chances that an Acer 5100-series laptop with an
AMD Turion 64X2 TL-56 1.8 Mhz processor with 2GB of DDR2 PC4200 RAM with a
533Mhz FSB and an ATI Xpress 1100 chipset with 256MB of dedicated RAM (as
much as 895 with sharing) will run Vista 64-bit decently, allowing heavy
multi-tasking and occasional gaming or multimedia?

I am thinking about upping the memory from 2GB to 4GB, and understand the
32-bit system will see no more than 3.12GB of RAM. However, am I to
understand that in order for the system to see the full 4GB when/if I go to
64-bit, that I would have to do a clean install, rather than an upgrade?

I consult and write about this stuff for the masses and I have to say I am
becoming more, rather than less, baffled about how we will ever actually get
to 64-bit Vista computing.