one of the main things to look at when you choosing a maker, is to find out who puts what speed memory on their cards.
take a gf4 ti 4200 64mb, for example... most brands will have 4ns memory on em which will give you 500mhz (2000/4=500)... that's the nvidia reference spec for that card. you may be able to overclock them a little higher until you hit the memory's limit at around 510-540mhz. but if you got the same card from gainward, for instance, they include 3.5ns memory on theirs.. officially good for 571mhz... up to 600mhz once you push it.
gainward also has a 'golden sample' version with 3.3ns memory on it but it costs more so you might as well get a 4400 instead. other brands also have 'performance' versions of their cards with faster memory and fatter heatsinks.. like that one from abit.