According to your hardware, you’ve got two main options:
– if your hardware (PC, Mac, etc) runs a 32-bit x86 processor with at least 2GB of memory, use Virtual Box; this is not the right option because you will not be able to experiment KVM virtualization and some advanced scenarios with 2 VMs but it’s better than nothing.
– if you’ve got a hardware running a 64-bit x86 processor with virtualization capability (Intel VT-x or AMD-V*) and at least 3GB of memory/50GB of disk space (2 guests plus 1 host, 1GB of memory for each), install RHEL 7 or CentOS 7 and use KVM (CentOS 7 is free, fully compatible with RHEL 7, and works very well).
My advice: buy a second-hand HP N40L (or HP N54L) with 4GB of memory ($200-250) and a DVD drive. It comes with a 250GB hard drive that is more than enough and you don’t need to add another network card. After passing your RHCSA/RHCE exams, you can reconvert it into a VMware ESX or a NAS (you get i/o at 100MB/s and can put 4-to-6 hard drives inside).
*To check if your Intel processor’s got VT-x virtualization capabilities, type:
# grep vmx /proc/cpuinfo
To check if your AMD processor’s got AMD-V virtualization capabilities, type:
# grep svm /proc/cpuinfo
Intel VT-d or AMD-Vi virtualization capabilities improve virtualization performances (they allow direct access to the hardware) but are not mandatory for KVM use (see wikipedia for more information).