I'm very sorry, but you can't overclock an OEM computer. They do this so that some customer doesn't go into the BIOS and screw with the FSB and burn their chip and then ask for a new one. Too many risks.
For the flashing: It depends on who made your motherboard. You might be able to flash it, but if it doesn't work you are screwed. And it isn't a hot girl screwing you. It's a fat chick who jiggles like Jello. Bad joke, I know, I know.
I know DFI made some OEM parts and you might be able to flash your OEM mobo to its retail counterpart. That might work, but again, if it doesn't work or something bad happens, you have a computer that will not function. At all. You'd have to get a new motherboard. Try explaining how you somehow accidently flashed a BIOS to Fujitsu, lol. I'm just kidding, but it isn't worth the risk. So you'd have to find out who made your motherboard, which is a difficult task with OEMs, look for its retail counterpart, if any, find the BIOS for it and cross your fingers. Good chance it will ruin it, but...
OEM computers aren't built for OC'ing in mind, they are to keep prices down. Chances are, there is a el cheapo heatsink on it and Value Ram (ugh!) and all those other bad-oc'ing parts. You would probably have to upgrade the PSU as well as they are barely powering what you have now.
Now there are some things called Clock Generators. You'd have to find some technical information about your motherboard, stuff they don't post on their website, imagining if you actually found the site for it. Something like the Clock Generator ID, can't remember what it is exactly. Then you have a GUI/Windows-based overclocker. It finds the clock generator inside the motherboard, the thing that calculates the FSB, etc. and is able to change it. Many OC'ers don't trust these programs, but you can give it a shot. Still, it could screw your computer, permanently.
Again, I'm very sorry, but you can't OC an OEM computer except for the "one-in-a-million" shots posted above.
One day you will learn all the rest of the evils imposed by the OEM manufacturers. OC'ing is an art, but whoever said Michael Dell was any good at art?