Two things - One, upgrade to ISA 2004 SP2 - there's an RPC patch that's
included that your x64 machines need in SBS Premium. If that's not possible,
see my blog post on it - there's a link to the patch you need there.

Second, since most SBS networks require the ISA client to work correctly,
with the default rules, you're out of luck right now - I don't think there
is a publicly available ISA client for x64. So, you need to create a special
rule for your x64 boxes.

1.) Give the x64 machines a fixed IP by using a DHCP reservation. This will
put them at a known IP address. If you have several, put them all in the
same general IP address range. On my network, I stuck them in .200 - .225.

2.) Create a Firewall Policy on the ISA server that grants the appropriate
outgoing access to all computers in that IP range. Personally, I just opened
it up fully to those boxes, since I knew who was on them and wasn't
concerned, but you can create one or more policies that are more restrictive
if you need to.