Depending on the rest of your computer you probably do not need a new PSU. 20 amps on the 12v rail is 240W, but of course that's probably peak and not sustained. Even so, a 5670 is pretty miserly and will generally use well less than it's max 75W (6.25A on the 12V) so long as you're not running furmark
. You should read this article on x-bithttp://www.xbitlabs.com/articles/cases/display/system-wattage.html
Anyway, it is a good idea to have a decent PSU, especially if you plan to upgrade your system latter or are concerned about your power bill.