Apps or websites? Those are two very different beasts.
If you're looking for app mockup tools, check this out. You could also just use Interface Builder.
Interface Builder sure is. The PSD files I linked to however were designed for Lion mockups. That said, I'm sure you could you could much of what was there to create Lion mockups since they share many of the same UI elements.
There should be a SL version of Xcode on the developer website. If there isn't, you can install it from your disk.
As for Lion, there are conflicting reports on whether or not it's a worthy upgrade. It's been a smooth process for me but I'm only 'one sound in a cacophony of different voices'.
It might not be. Remember though - you can get it off of your install disc.