You want to focus on the amount of memory, processing cores and DirectX version first. Clock speeds are important when trying to compare similar cards. Don't really do a whole lot by themselves.
i would make sure you get a nvidia 8600GT or higher, if your looking at ATI get an HD2000 or higher
if you get something lower, its not gonna last. my 8600GT dips to 40-50fps when i'm playing sins of solar empire on high with af and aa and my card is fast as f*ck (core 850. shader 1600. memory 1450.)....
youll be wishing you spent the extra dollars when you cant play dead space 2 cause your at 14fps on low