Basic Postgresql Performance Question

Hi,
I currently have a mysql server running with a database of around 800
gb.  The problem is that the server is old (500 MHz Pentium III with 512
MB RAM) and I want to change this to a new server and convert the
existing database to Postgresql on Debian (I assume that Postgresql
offers better performance for complex read only queries on large
databases), though I was wondering if

1.  It is possible to have some sort of load-balancing through buying
many computers without replication, i.e have one server which has the
databases and then other servers which has no database but just exists
to balance the memory and processor load? (I have heard this is possible
with C-JDBC)It is difficult to have enough space to replicate a 600 gb
database across all computers)

2.  It is advantageous to buy AMD 64 rather than the Pentium IV?

Any thoughts?
Thanks.