You should be able to get it to 3.0 Ghz without Vdimm bump, http://xtreview.com/review166.htm , anything higher looks like a challenge. The 12.5 hammer is the hold back, you're gonna have to go old school and crank the clock. And the article sez you'll do better with a reduced mem ratio, but remember all dies ahve different caps, though ther's general range with AMDs.
Need to learn the basics? Here's a super reference: http://hardwarelogic.com/news/127/ARTICLE/2646/2008-02-01.html
Good luck, keep the toast in the kitchen , and out of the motherboard. 
Penewah!
Saving for the 4870X2! MSI K9A2 790FX / AMD Athlon 64 X2 6000+ Windsor 3.0GHz/Xigmatek 120/ Antec 900/HD4850 Xfired/mushkin Black CL4, 2 x 2GB DDR2800 /Samsung Spinpoint 320GB SATA/ PCPC 750W CF Silencer/ LITE-ON 20X DVD±R /WD My Book 500GB/Logitech X-530 70 watts RMS 5.1 Black Speaker System /Build2: Biostar 780G/AMD 4600+ x 2 @ 3.0/OCZ Platinum Rev2 2 x 1GB DDR2800 /Logysis Acrylic/Vista/Ubuntu 8.1 / Corsair 550W PSU