I was able to push to a 378 FSB without any voltage changes but your memory might not take it with the memory freq set to 1333. You have to decide what tradeoffs are acceptable but I'd try going up to a 378 FSB and lowering your memory frequency to 1066. This will put you at 3.4GHz for a processor speed but a slightly underclocked memory speed of 1210MHz.

If you're worried about your array, burn a Memtest86+ CD and use that to do an initial test of new settings.

I also created a version of the XBX2 memory calculator for the X38 so you can use that to see what happens to your memory at different FSB speeds.