I haven't seen or heard of any oc'ing benefit from having it turned off or on.
AFAIK, there are not any apps per se that use it ... it is really to enable the running of multiple os's concurrently on your pc using software like vmware.
Many folding folks used it and the free version of the vmware server to enable them to run the 64 bit linux SMP folding client initially, before stanford released their win smp client.
So it is a pretty nice way to be able to run linux at the same time as wxp without needing to use dual boot.
And for those folks that run native linux, there is a version of it that lets you run windows under linux!
If you have enough resources, you can run multiple os's under it at the same time.
vmware i think helped create the market to some degree ... many large companies were looking for ways to reduce the number of physical servers they needed to buy and operate for their hundreds, or even thousands of 'standalone' windows and linux based servers apps. MS's Windows datacentre edition has only ever been used much for very large scale single mission apps, as opposed to the mainframe world where the norm is for many different apps to run on the same os and hardware.
There are similar things happening in the Unix server marketplace, but outside of vmware and linux, I think most of them are based on the various proprietary Unix's (and probably proprietary hardware) and allow for consolidation of solaris or aix servers onto larger scale hardware.
VMware has some pretty cool features when you are running the enterprise versions (which cost 10's of 000's) ... you can move a running virtual machine from one machine to another while it is running! ... clone images to give developers their own virtual dedicated environment in minutes, instead of hours or even days of software install work. Lots of efficiencies for big companies (like the one i work for).
Some apps don't lend themselves to vmware style sharing because of things like very heavy database usage, network usage/config's or poorly coded apps that depend on low level software interfaces (ie below the virtualization layer). VMware provides a virtual bios. VT basically provides segmentation support. Newer AMD's also support it. It isn't very suitable for high performance video intensive workstation apps because of the extra software overhead.
The free vmware is pretty easy to setup so give it a try. Here is a link to settting it up + ubuntu for folding
Beginners guide to VMWare and SMP install - Overclockers Forums
and there are other similar guides around .... that thread has links to some of them.