|What are you going to do with it?|
Two things tend to speed up computers.
One is more ram if you can. Be sure how much your system supports. No need to waste money on parts that won't work.
Second is faster hard drives.
Really third is graphics if you use it for graphics. This goes back to what are you using it for.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10