You mention that you have a MacBook Pro, so what exactly do you use the old Dell for? Just as a portable computer for traveling (while the MacBook stays at home)? If you are just using it only for things like e-mail, web stuff and looking at pictures/video, then a light tablet such as an iPad 2 or the new Samsung Tab 10.1 is probably best. With a good folding case (both tablets have excellent cases available) they are easy to carry around. A 7 inch tablet might be better if those are the sorts of applications you need it for since they are easier to slip into bags than the bigger ones, allthough I don't think there are any yet with slim profiles (Samsung are supposedly coming out with smaller versions of the Tab 10.1 though).
If you also need it for any sort of application software (such as wordprocessing, spreadsheets or presentations) then go for a 14 inch Windows laptop. They are reasonably cheap and powerful. Pretty much any current laptop will kick the snot out of every tablet in terms of performance. Tablets that are currently on the market are basically giant smart phones without the phone part.