rc1 upgrade over pre-rc1 fails

Here's a weird one, I'm totally baffled. I originally had pre-rc1 installed
on a nforce2 board. In prep for rc1 I upgraded to a nforce4 w/a64 3500 (I
want to test both x86 and x64 versions). vista had no problems detecting the
new board and seemed to work the same as ever. However, on dl'ing rc1 and
trying to upgrade it fails saying I need 600meg free. Ram usage is at 26%.
Now the odd part, looking in task manager it says free ram is 6 meg. Using
sysinternals process explorer, it says I have 1.5 gig free. So, where is all
my free ram going? I'm assuming it's going into "cached memory" for some
reason since task manager reports:
Physical Memory
Total 2046
Cached 1615
Free 34

Another oddity... page file 2221m/2993m

Anyone else seen anything so weird?