If "extreme" is your middle name and you have can afford it, an Athlon FX-57 / Phase Change system will last you a few upgrades. Even though the FX-57 did very well with the sub zero cooling, the more potent Prometeia Mach II GT or even Vapochill LS (which get about 10'C colder) it has an extra few MHz in it.
I was able to perform a number of benchmarks at 3.54GHz as well, but the Mach II just didn't seem to be able to hold the colder temps under full load and the longer benchmarks failed after a while.
Looking back though on the performance increases from 2.8GHz to 3.45GHz it ranges from relatively small to enormous. The smaller increases come where the graphics card plays a bigger roll such as in 3DMark05 - but still there is a noticeable increase. The biggest increases come where CPU power is the main factor. Here one of the most noticeable increases is in Quake III at 800x600 where there is a massive 29% increase in performance.
Also the increase in the gaming benchmarks would have been much more - because believe it or not - at these kinds of speeds, even the mighty ATI Radeon X850XT Platinum Edition becomes a bottleneck.
The huge increase in memory bandwidth did play a role as well. With motherboards capable of pushing such huge amounts of voltage through your memory, it seems a shame not to put it to good use. Two models currently available are OCZ's PC-4000 Gold VX series and Mushkin's Redline PC3200 series which we recently reviewed.
With the push towards Dual Core processors well under way, it doesn't seems likely that we will see a Single Core AMD processor running at 3.45GHz anytime soon. If that day does come the chances of us still running DDR1 on our AMD systems is highly unlikely as well. Until then, those of us that are not satisfied will always find ways of making the fastest even faster.