If you up the voltage, you're going to give that CPU more power. Effectively that CPU is going to heat up more. I'd probably do one of two things. Clock it down and lower voltage or I'd chump up some change for a new heatsink/fan. I never had a lot of luck with the Zalman series. I've had a 9500a floating around here that I actually used for a while with my 2600k. But it's not a "Good" heatsink but definitely was better than stock.http://ark.intel.com/products/35428/Intel-Core2-Quad-Processor-Q9650-12M-Cache-3_00-GHz-1333-MHz-FSB
That's intel's rating for the processor... 71.4C. I'm not sure the exact MAX of the processor but, I'd definitely back it down until you find out how to get that temperature down.