will my PSU handle an external graphics card?

"For future reference on the topic:

http://educations.newegg.com/tool/psucalc/index.html

A handy PSU calculator courtesy of newegg.com"

It probably will not handle any current video card. That's not to say that maybe you couldn't find an older card that would work with your psu, perhaps an older pci. I doubt it will have the performance you are looking for though. Power supplies are fairly inexpensive so upgrading it usually doesn't kill your wallet and it's a good place to plan for the future also. The more steady power you have the better. Hope that helps.

4670 is rated for only 59 watts max. It's a very efficient card.