How about keeping your old phone and use stand alone slate like the Archos. The Galaxy Tab is rather redundant if you already have a working phone and you'd end up paying for two phone packages.
On the other hand you can just get a non-3g slate and connect it to the phone via bluetooth.
So the thing you have to ask yourself is if you want a phone tablet that can do everything in one package? or a multimedia one that can tether to your existing phone anyway (And not get awkward holding a clipboard to your face)
And regarding the resolution.
http://www.youtube.com/watch?v=zasSIA7sexM
It's not really an issue since most apps are designed to fit any of the myrad of cellphone sizes. There are some apps that are specifically designed to work with a single resolution and the android specs says that the device is supposed to add black bars around it. The fix simply tells the OS to ignore specs and resize the app anyway. Worst case, this gets the same effect as an iPod getting force fitted on an iPad.
And one of the commenters is an Archos user that apparently implied that his tablet has spec compatibility disabled by default.