x2 as x6 multiplier only works in k6-2 400 with stepping 8 or higher (just check www.amd.com) and ALL k6-3. So, your k6-2 350Mhz don't have it. Also, older non super7 motherboards can hold it. I have an Asus SP97-V with an unsupported 83Mhz than run smoothly, an even the official manual don't say anything about k6-2. So, surf the net and find others. In my case it have a k6-2 500 and I use the x6 trick (83x6) to reach 500Mhz :-)
As people say, check dual volt and compatibility issues before you buy any MB.
DIY: read, buy, test, learn, reward yourself!