I use Virtualbox for a lot of desktop related projects. Although it is not really the first choice for server related virtualization projects, sometimes you might just need to proof or test something quickly. Here is what I did to enable the guest additions on a Redhat server running as a Virtualbox guest.
Note since I did not want to waste an entitlement license on RHN I installed packages from the DVD. Yum can do a group install for the Development tools which would be easier.
Install some compile tools:
[root@localhost Server]# rpm -i glibc-headers-2.5-105.x86_64.rpm libstdc++-devel-4.1.2-54.el5.x86_64.rpm gcc-4.1.2-54.el5.x86_64.rpm gcc-c++-4.1.2-54.el5.x86_64.rpm glibc-devel-2.5-105.x86_64.rpm
Install kernel headers:
[root@localhost Server]# rpm -i kernel-devel-2.6.18-339.el5.x86_64.rpm kernel-headers-2.6.18-339.el5.x86_64.rpm
Install guest additions:
[root@localhost VBOXADDITIONS_4.2.6_82870]# ./VBoxLinuxAdditions.run Building the main Guest Additions module ...
That was it and I could use copy/paste, better screen resolution and share files from the local host.