Your X-Box drive has free space? You need more games
Recording hours consider that the 160G is 4X larger then the 40G so probably a little over 44 hrs (11hrs * 4 ).
Should not be a problem using the 160G in the TiVo, be sure the drive is unlocked before you remove it from the X-Box. Most first time modders keep the original TiVo drive as a backup to put back in JIC the TiVo needs replacement later on. Large IDE drives are in-expensive now days. At the minimum make a backup without recordings then test it on the replacement drive before re-using the original drive.