I'm not sure what your saying with having taken the memory out of your 360 and having put it in a USB case. But if you do the towel trick on your old 360 you should have enough time to transfer all your saved data onto a usb which you can then use on your 360 slim. Although I think just buying a transfer cable would be much less hassle.
Also I have no idea what your talking about voiding the 360 slim warranty by removing the hdd, those hdd's were designed to be removed so you would not void the warranty by removing it. Also yes you can get the 360 slim arcade and add extra memory later on. If your really tight on memory you could just add two USB's to your 360 slim and that will give you an extra 32gb of space which should be pretty decent.
The definitive evidence that video games turn people into mass murderers
