@overclocking: usually not a problem for both AMD and NVIDIA ... you don't need to pay more for a vendor-overclocked card, as you can usually freely adjust the clock speeds with software. the power/heat/performance are scaling accordingly. For example, i've used this with the AMD cards last year to use exactly as much power as my photovoltaic panels were delivering. i simply polled the produced wattage from the solar panels every minute and adjusted the AMD's clock speeds accordingly. depending on how well you can keep cool your card, and as long as it's stable, you can overclock.
@different brands: well, first we have to differentiate between OEM design cards and reference design cards here:
OEM design cards: e.g.
http://gzhls.at/p/1186788.jpg - you do not want this. the coolers are AXIAL. typically two or three.
REFERENCE DESIGN cards: e.g.
http://gzhls.at/p/1167949.jpg - this is the type you want, the cooler is RADIAL. typically only one. sucking in the air at the rear, and blowing out hot air at the slot bracket.
Do not get mislead by the fact that the reference cards "only" seem to have one fan. that one fan is very effective, usually a lot better than the OEM fans. Especially if you have more than 1 card, you will soon get into troubles with OEM cards. OEM cards will typically cycle the air inside the computer case. it will warm up continuously. - The reference cards are different, they're blowing the hot air outside the case. that's a lot better. You can put multiple cards together just besides each other, too. - something that's typically not possible with OEM cards.
As usual, there are exceptions: For AMD hd7990 even the reference design used AXIAL fans. also, i have indeed experienced one card that was hotter than the other two. even though they were reference cards looking and built exactly the same way, just rebranded by sapphire and asus. i refreshed the thermal gel on the hot card but still there was a difference in temperature (before: 10 hotter, after changing the thermal paste: 5 degrees hotter). i still don't know why, but this seemed to be an exception, normally reference cards are very similar if not even the same, possibly just rebranded.
if you are looking at different OEM cards, there might be bigger differences because every manufacturer has different cooling fans or did other design changes to the card. usually you don't want these OEM cards, though. You really want to stick to the reference design cards. usually they are built exactly the same way, or at least 99% the same way. at least that's my experience so far (epixoip please correct me, if i'm wrong)
