As a K100 owner and long-term Pentax user I'd happily recommend it, but I don't think you can say it's much better than either a XTi or a D80... not without being eaten alive by the Canon & Nikon folks anway.

The K100 is small and fairly light but still feels well-built, the design is very good, and the anti-shake is useful... but in terms of the images captured by the sensor it's basically comparable to a D50 with the addition of 3200 ISO. That's not a bad thing, but it may not seem that great if what you're after is the 'next big thing' or more megapixels for the money. IMO the K100 is more comparable to the D50 and Rebel XT rather than the D80 and Rebel XTi, so it's difficult to say it's better than the latter... the new K10 on the other hand is a different matter entirely, with its 10mp and weather-sealed goodness...
Anyway to reply to the original question... if you've never used a dSLR before then
any of the current or previous generation will be a significant step up from a point and shoot. If cost isn't an issue then you can afford to go for a newer model, but more importantly you can afford to buy good lenses. Glass is very very important. A really good lens on a 6mp dSLR will produce better results than an average lens on a 10mp model. Of course if you can afford the latest dSLR model then go for it, but understand that to have the range of lenses that gets the most out of that camera may cost several times more than the camera itself. Lenses, they're important. Did I mention that already?
With that said, if you have no specific brand preference and no previous experience with film or digital SLRs then you are free to consider the offerings from Canon or Nikon and also Pentax or Sony. Honestly, they all make good cameras and good lenses are available for all. A lot of people will talk about what the professionals use, or how much each company spends on R&D, but you're the one who has to use the camera and you have to feel comfortable doing so, so I really think the best way for you to make this decision is to go to the shop and try out the different models.