it depends on the series, brand or manufacterer.
generally, the higher tyhe number the better.
for instance, in ati cards, the first number is the series: 5xxx is a 5 series card.
58xx is better than 57xx and a 5870 is better than 5850.
with nvidia, its virtually the same with a 9500gt being better than a 9400. but more letters is good too, with a 9800gtx+ being better than a 980gt. it then also splits off though, with thew high end cards being '200' series. GTX260 or GTX 285 for instance.
it ca be a little confusing, are there particular cards you are looking at?
No, I'm not looking at any particular card. Something around 100-150 $. My main problem was that I just didn't know how the names worked and it was causing problems.