Not all phenom's but lots in general are very hard to get stable with high HTT speeds. We're talking 240+ can be unstable on a lot of boards. I'm not sure how much SB750 improved things as most people playing with phenom's and clocking have unlocked editions.
Early indications are the p2's are clocking far better on the HT side of things. My personal experience was 3Ghz was easy with a release week 9600 BE, but only with multi anything over 220fsb was a no go on it.
Personally I'd say spend the extra on the multi, higher HT speeds aren't doing a huge amount for speed, higher northbridge speeds are, though I'm sure you can use a multiplier to knock them up/down.
I don't think it will be a huge limiting problem at least on air maybe. If you're looking at 4.5Ghz on water a 920 might stop you getting anywhere near that.
What AMD really need to do is go back to all chips being unlocked, make the black edition just the highest clocked version and let the few people who buy just to overclock(which is a tiny portion of the market, but also by far the most vocal and the people others tend to ask for advice on computers being the key thing) buy a £100 quad that will clock freely as well as their top end chip like they used to.
At the moment, a 920 i7 isn't a cheap setup, but the fact the chips are similar price won't encourage too many people. If you could get a £100 chip that overclocked the same, they'd get it, overclock it, save a bundle be incredibly happy and tell 50 people to get one too.
But they'll only be getting 1, I want more 
