.
I run Windows XP in a VMWare guest on a Kubuntu host. While I don't run VirtualBox, the principles are the same. Some thoughts and comments:
** Be careful to not starve either the guest or the host of RAM. If either one starts paging virtual memory to disk, it will affect performance.
** On my own workstation, I run the Windows XP guest in VMware with 512MB. The rest (1.5GB) goes to Kubuntu where I spend most of my time. I get very snappy performance out of both, but I also have a dual-core CPU.
** Your initial idea (768MB for XP in a VM, 256MB for Ubuntu host) might starve the Ubuntu host (running X) even if you don't run any other Linux apps except VirtualBox.
** If you will spend all of your time in XP while the VM is running, you might get away with it. Try it and see. In Kubuntu, I would check memory usage with KSysGuard (similar to Windows Task Manager...if you are not familiar with KDE).
** With "just" 1GB, I would start with 512MB each for host and guest and see how it goes.
** If I were you, I would look to upgrade to 2GB. With 2GB, you would certainly have enough for both host and guest to perform well with running a "normal" set of applications in each. Many types of RAM are cheap these days (but not all). Your AMD 64 2800+ PC probably uses DDR2 RAM. DDR2 is very cheap right now:
DDR2-800 PC2 6400 2GB (2x1GB) $60! ($50 after rebate)
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2+50001183+40000147+1052416064&name=DDR2+800+(PC2+6400)&SpeTabStoreType=1
Hope this is helpful. Good luck!