If you currently have speed issues with your machine, you should read through my FAQ*:
http://www.macmaps.com/Macosxspeed.html
An unhealthy system does not bode well for the ability to upgrade your machine. Upgrades generally add features, and don't fix issues that build up because of data corruption, a machine that is overfull, lacks resources, or has third party utilities which haven't been properly tested with the system which can all slow down your machine.
- * Links to my page may give me compensation.