There is a usb key with a e-ink screen that shows the used space. I just don't remember where I saw it.
Ok, something like this http://www.lexar.com/products/lexar-echo-mx-backup-drive?category=207
HD 6870 price
Check this page: http://www.compusa.com/applications/SearchTools/item-details.asp?EdpNo=6799926&CatId=3585 according to that the 6870 is gonna be around $270. It lands in HD5830/GTX460 zone.