I have been tasked with the same problem and I installed Oracle VirtualBox on Windows 7 (as host) and Ubuntu (as guest). I used a standalone laptop, HP EliteBook 8540W with Intl Core i7 running at 1.87 GigHZ and 8Gig RAM.
Assuming your installation works properly, on your Guest machine (e.g. in my case, Ubuntu) select "Settings..." from "Machine" in the main menu; select "System" and click on "Processor" tab.
You can adjust the clock speed (labeled, "Execution Cap") by sliding the indicator on the bar all the way down to 1 percent.
I assumed that the host OS (Windows 7 in my case) was not consuming any clock cycles (I confirmed this by checking the "Task Manager" and I found ZERO CPU utilization on the host machine .i.e. windows 7 in my case). Therefore, I asuumed 100% "execution cap" equals that of host machine (e.g. 1.87 GigHZ). I think by taking a percentage between your host machine clock speed and 800 MHZ, you are able to figure out the percentage value that corresponds to 800 MHZ.
I hope this helps.
Best of luck.