Mixing Different Brands of RAM

On an old system I built I have 2 gigs of Crucial Ballistix DDR2 PC 6400 4-4-4-12 at 2.2v. I wanted to upgraded to 4gigs, but, much to my chagrin, Crucial has switched to a new model that run 4-4-4-12 at 2.0v. Since I can't find the old 2.2v model at the price point and quantity i want, would it be okay to switch to a different brand of RAM like Patriot that runs 4-4-4-12 at the same 2.2 voltatge and mix sticks from two different manufacturers ? Or should I buy the new model of the Ballistixs and overvolt them ?
I've always heard that you should use the exact same RAM when you upgrade.

Some motherboards are very sensitive to mixed ram. Even the same spec ram from the same vendor can cause problems because the components can be different from run to run. That is why it is best to get a kit of 4gb.

You can get a corsair 4gb kit for $25 after rebate:
http://www.newegg.com/Product/Prod [...] 6820145184