That is something that cannot be summed up into just one product when it comes to GPU cooling.
If you want the absolute lowest temps (on air) the the Thermalright is a perfect choice. However, it is almost ridiculous how much space it takes up and how heavy it is, especially with a fan attached.
The Arctic Cooling Accelero S1 V2 is also a very good choice when it comes to having the coolest GPU, and with the Turbo Module (the fans,
) installed it still takes up less space than the HR03GT. In a lot of reviews I've even seen this sucker cool absolutely on par with the much more expensive Thermalright+fan.
From what I gather, the Zerotherm Hurricane HC92 Cu is quite a good cooler as well, but it's also a behemoth.
If you have to have a 2 slot cooling solution though, your choices are quite limited.
The Thermalteke DuOrb would be my first choice here as it has proven to supply greatly increased cooling over the stock unit at a (now) low price (about $35) and it looks trick too.
The Zalman VF1000 or newer GV1000 is a very good unit as well, but it will cost you more than the Thermaltake eventhough it will run around 1-2C hotter than a DuOrb in some tests that I've seen. But that margin is negligible and if you like the Zalman's look better then it's a solid choice.
For the 8800 GT in particular the EVGA Akimbo cooler is also a very good solution, as it really helps cooling over the stock cooler and it's a breeze to install too.