HELP: How to optimize use of RAM in Windows XP SR3?

I have 768 MB of RAM. How do I optimize its use for VirtualBox on Windows XP, SR3?

As I proceed with the installation of my VB guest OS, GuestBox seems to need more and more RAM, until it eventually aborts. This is a repeatable situation.

1) Is there a way to cut the initial allocation of RAM for the host operating environment (XP) and the guest OS (ububtu 8.04)?

2) How do I decide how much of the 768 MB is available to VirtualBox?

3) How do I minimize demands for RAM?

I have decided to up my total RAM to 2 GB, but this takes time, so I am looking for a short term solution.