|Well for one thing don't trust the new 2 TB drive untill you have tested it extensively or have used it for at least 3 months to a year. Apparently about 10% to 20% of these newer large drives fail in the first 3 months to a year. Either way if the contents of your 160 GB drive is important you should have an image of it on another drive in case you suffer a failure. One FREE and excellent piece of software to image your drive with is Runtime Software's DriveImage XML. You can image a drive right from windows without having to go into DOS mode. It's also only a 3 mb download so it's very efficient and easy to install. You can also get a copy of Hiren's Boot CD which has a few utilities for duplicating your HD, including that one.|
Windows boot times and performance does depend on the drive to some extent, particularly if you don't have enough RAM and the swap file is being used a lot. You can do a quick speed test on the drive using a free program called HD Speed, or another one HD Tune, and you can run extensive tests on the drive using another free program called HDDScan. This program can run a test called a "butterfly read" which will take about 19 hours and will make the drive read data randomly from the whole drive which will exercise the mechanism and help ensure your drive is working well. It will also show you the SMART data which you can save to a file and compare to later results and see if the drive is still doing well.
You can actually use two hard drives for keeping a backup of you data. It might not be the best idea to trust two internal drives since a bad power supply or lightning damage can blow both of them but generally it's better than not having a backup at all. Otherwise it's quieter and more power efficient to only run one hard drive in your computer. So if you keep a backup and test your new hard drive, then sure your computer will probably run faster off the new drive than the old one. You can even mount your old drive in an external USB enclosure and turn it on daily to do a backup.
Hope this was informative. Feel free to ask any questions. Good luck!