Hi IckyThump,
I've just bought the Sigma 17-70 for my Pentax K10D and I can tell you it's a cracking lens for the money. I'd agree with Grendel that, given your budget, it's got to be worth a look. The only thing I would say however, it's quite a bit heavier than the kit lens of the 400D (if that's what you're using now).