I recommend you to practice in KVM instead of VirtualBox. It is in RedHat exam objective that you should be able to work on KVM, atleast in a limited way. Particularly in RHCE exam, the servers you setup/modify are KVM virtual machines.
The reason i'm telling you is, four of my friends took RHCE last week. Two of them took Redhat training (2 months) at Rs.15,000/- (excluding exam voucher). Both passed. Other two, who have atleast 5-8 years experience in solaris, but did self-study, only passed in RHCSA and failed in RHCE. Only reason is that both had over confidence, and when presented in KVM, both got confused and eventually failed.
Strangely, when i asked about exam setup, they told that "there were physical server attached to exam machine, but were connected through kvm
"...Which i didn't understand. Probably they mistook it for KVM switch.
Incidentally, i'm also taking RHCE exam (having Windows Server background) by December end. If you take my advice, setup CentOS as Base Machine, install KVM, and in that install CentOS/RedHat virtual servers to practice on. While you are in Base machine, instead using Nautilus and other GUI tool, use terminal for all jobs.
Also if possible buy 'RHCSA/RHCE Red Hat Linux Certification Study Guide (Exams EX200 & EX300), 6th Edition (Certification Press) by Michael Jang'
Set up RHCSA/RHCE Lab with CentOS
Best of luck.