Hello, I am running XL 2007 on 512 MB of memory, and it runs fine.
I also had XL2003 when I upgraded to XL2007. I chose the custom installation and selected the option to retain previous versions. The only problem I encountered was when I opened an XL file that was created by the 2003 version, the 2003 version of XL opened up instead of the 2007 version. I wanted the 2007 version to open when I clicked on any XL file. If the XL file was created and saved by XL2007 version, then the 2007 version would open up.
That was the only problem I had with both versions installed at the same time. Once I became familiar with XL2007, I uninstalled the 2003 version.