As to how far they'll OC, it depends on many factors, but from what I've been reading about the Venice cores, 2.5Ghz would be a fairly easy to hit target. Of course you may be unlucky and get a poor overclocking core, or you may not.
For instance, I have a Winchester 3200+, and so does Ned_Flanders (another user here). I had little or no trouble getting mine to run at 2.6Ghz with the stock cooler, whereas he had a real nightmare trying to get over 2.5 or so, and he's using water cooling. The newer Venice cores seem much more consistent overclockers than the winchesters though. Many people are getting 2.7Ghz and above.
If you can find out the stepping code on the CPU before you buy, post back here and someone will probably know if it comes from a 'good' overclocking batch or a bad one. I have an Athlon XP1700+ that comes from a renowned good batch, which is why I bought it to replace a 2000+.
So in other words, you never know until you try, but it's the trying that's fun.
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<font color=red>"Life is <i>not</i> like a box of chocolates. It's more like a jar of jalapeńos - what you do today might burn your a<b></b>ss tommorrow."