Yes, it is true but it does depend upon what you uninstall. Not everything will impact upon performance if you remove it from the equation.
As for what to remove, that's simple: anything you don't use, will never use, don't want.
Personally, I always start with the Internet security suite as the ones that have done deals with the hardware vendors have a tendency to be true bloatware and can really hit performance hard. I have taken down McAfee and Norton from numerous Windows 7 machines bought by relatives and friends, and replaced them with Microsoft Security Essentials which is free and does a decent job of protecting the PC without slowing it down so much.