The difference between cards in the same series or model can be seen a little. It comes down to different cooler designs, different PCB designs and some have factory overclocks which you can pay a premium for.
For example, the
ASUS EAH5770 has a different/bigger cooler and different PCB design than the
XFX HD5770 Single Slot I run in my machine.
My card has no overclock, but it is smaller than the average HD5770 and that reduces costs and I bought it for £85. But a HD5770 that is factory overclocked and has a bigger cooler on it may cost an extra £10.
It is stupid, but sometimes you can get a card that has the potential to be overclocked that much its performance can be boosted by 20% i.e. MSI
GTX460 HAWK. This card is the most expensive version of the GTX460 around @ £140 but with the massive cooler and the circuitry in the PCB can allow overclocks of up to 1000mhz on the core clock and 2000mhz on the memory clock.
A normal GTX460 i.e.
MSI GTX460 Cyclone at £127.99 will only just hit 860mhz on the core clock, maybe 900mhz and will have no chance to hit 1000mhz because it has not been designed to do so, unlike the
MSI GTX460 HAWK.
I would not pay for factory overclocked cards as they usually just add an extra 25-50mhz on the core clock and this is at a price. My HD5770 is standard 850/1200 but I have overclocked it stably at 920/1350mhz. That's 70mhz more on the core clock and 150mhz on the memory clock, which has resulted in dirt 3 having an extra 9fps which is very noticeable on my system with my overclocked AMD Athlon X4 640 @ 3.4ghz from 3ghz stock.
Hope this helps!