Vista needs 2GB to run efficiently. You probably will end up paying twice as much through the manufacturer than you would through a retail store (like newegg). So avoid loading up on RAM when configuring it. Get the bare minimum unless there's a deal, then upgrade later.
Whether you choose a Turion X2 or a Core 2 Duo, consider price-to-performance first. A Viper might be faster, but it's also very expensive. A Mustang would give you better performance for your buck. So don't count out AMD. But since laptop processors throttle down when you are mobile, raw processor power is probably the least of your concerns. Generally, you will find much better performance in a Core 2 Duo notebook, but this comes at a higher price.
Build quality is very important. Dell is known for having poor build quality, at least in their consumer line. HP was your second choice you said. It has much better build quality (and much better design IMO). Even better are HP business notebooks (Compaqs). I've also heard Asus makes solid, durable notebooks.
Since you may be using this in an office, you will have to consider screen types. There are two: Glossy and matte. Matte is good for high-light conditions, since it doesn't reflect light into your eyes. The colors are arguably less crisp- but it's probably the choice for office or outdoor environments. Glossy is good for indoor, low light use. Since there is no matte finish, the colors look more alive and crisp- but they reflect light and could be a distraction.