For a drive that size, definitely partition. It doesn't kill performance, it actively improves it. The fastest part of any disk is right at the beginning and can be up to 20MB/s faster than the slowest part (average is about 55MB/s for a standard 7200rpm).
Set aside about 15-20GB for windows and key programs and let the rest use the other partition. This also means faster defrags and less defragging of data.
It can kill performance because seeks might require the head to move from the windows partition to the other partition which will be 20GB further out than it might be - so there are disadvantages too to be aware of. For such a large drive, the benefits outweigh the disadvantages.