I'd gladly pay $150 for a mobo and gain a whole 1000MHz for it. Not to mention you gain much more than just CPU MHz when going to a new mobo, you get new supported features, faster bandwidth, more features and faster all around performance.
Chances are if you didn't buy your computer in the last 6 months...it's beyong obsolete. AMD is already phasin out the ancient XP line of there chips and deaming Socket 754 to only last another 2-3 yrs.
DDR2 isn't looking too good for AMD right now either, people seem to be sticking with DDR for it's timings and speed. DDR2 hasn't been deamed overclocking worthy in any aspect as fasr as i know...i wouldn't buy it yet. But it may become more popular as it gets better...its still pretty new.
Again i think PCI-E is still new...and totally under-developed. It's not showing significant speed gains over AGP8X...at lest not enough to turn heads. IT's only popular because it's new and flashy. SLi is also pretty worthless and hopefully will be phased out soon...it was just an excuse for not making dual core cards lol.