The system requirements are

NVIDIA® GeForce® 7800, ATI Radeon™ X1800, Intel HD 3000 or better (256MB of video RAM and shader model 3.0 or better)

I have a Radeon HD 6670 1 GB

will this be enough?

How can I tell if a card is better then what they list?

Is there a website I can go and compare two cards to determine which is better?

You can test your computer here. You will have to install a browser plug-in.

Usually cards are measured, at their most basic, by the amount of video memory.

Thank you that website is pretty good. I wish there was an option to enter my computer system specs without having to download and scan.

It's more surefire with the plugin, that way you don't enter something incorrectly (though I will say that the SysReqLab site does go a little funky reading on-die graphic chipsets). I don't know what proc you are running, but your current GPU will cover the minimum requirements listed for Guild Wars 2. I don't have much experience with the game, as I don't dabble in MMOs, so I am just going off the game's website.

My old Geforce 9800 GTX 512Mb gets decent framerates in GW2 at medium-high settings, so you'll probably be alright. Might not be able to max everything out, but you should be able to make it look good and still have it be playable.