By "limit", I meant going on people's experience with the Pentium 4. Those CPUs cost big bucks when they came out because it was so hard for Intel to get decent yields at that speed with that design. Even on the last revision of the P4, the fastest version sold was 3.6GHz (I'm not sure which revision/version you have, but I suspect you have one of the earlier ones that maxed out at a 3.4GHz model).
MBs can often be run faster than their specs, as sometimes the spec just means that Intel hasn't released a faster CPU yet. When they do, the MB will run that, too. However, your MB is made by a company that makes lower-capability MBs, using a chipset made by VIA, which is known for cheaper, lower-capability chipsets that are normally not very OCable. Cooling can only take you so far -- it can't overcome limitations in the design.
As long as you don't boost the CPU voltage or let the CPU get too hot, I'd go ahead and give it a try. It's only overvolting stuff or letting it get way too hot that will damage it.