It looks to be about 2.8ghz, which should be fine for L4D. I put CS:Source on my laptop, and it runs at 60fps with full detail, on a 2ghz processor.
Thing is - people think that if your PC is not multi-core, then it's slow. A lot of older games, a lot of new games, in fact most games will not take advantage of multi-cores. So, really if you take a dual-core processor at 2ghz for instance - the chances are that your processor would out-perform it because L4D won't be using that extra core for anything.
Maybe there's a demo to try to be on the safe side - but really I don't think there's anything to loose - L4D will be cheap on Steam about now, it's worth the gamble to get such an awesome game.
