Scratch disk is just one area to set for optimum PS performance. http://helpx.adobe.com/photoshop/kb/optimize-performance-photoshop-cs4-cs5.html
Depends on how big the file is ... If you are not running out of RAM then no. Always add more RAM until maxed out.
When using Pshop see how much RAM you're using (e.g. CTRL-ALT-DEL on windows) - if you're maxing it out, buy some more (RAM costs almost nothing nowadays). If you can't add any more RAM and you're still maxing it out - then scratch disks become useful. They generally work best if it's a separate drive.
