How can I tell if it is stock or factory overclocked? and how can I find out what kind of cooling it uses?
Last, does the amount of gigabytes mean anything? I read somewhere in an article that there were other more important aspects of a card than simply the amount of gigs advertised.
the stock clocks on a 5870 are 850/1200. The reference cooling looks like this http://mos.futurenet.com/techradar/Review%20images/Tech... if it has a different type of fan on it then it is a third party. 1GB on the cards is going to be enough unless you are going to be running 3 monitors @ 7680x1600.
One difference specific to the HD5870(and HD5850) is that some of the cards allow for voltage modification of the core and some do not. If you are going to overclock the card then it would be a good idea to get one that allows you to bump up the voltage. This should allow you to achieve speed between 1000-1050 mhz while the stock voltage will likely limit you to an OC around 900-950 mhz.
FYI from looking at the customer review that XFX card does not allow for voltage modification.
It's pretty easy to do and there is no real risk involved but in your case you probably don't need to be concerned about which cards can voltmod. You'll still be able to overclock if you choose to do so, the other cards will just allow you to push things higher. The Catalyst Control Center itself can be used to achieve a mild overclock very easily.