I am looking for a new Graphics Card, today I have a 4750 with a Artic Silence cooler and a 120mm-fan, the fans runs at 400-500 rpm. This is much more than needed to cool the card it turns out. Would a similar solution be enough for a 7870? Or do you need to go down to 7850 to run such a setup?(or eventually use 2 fans, or a AC-solution with the triple-fan-thing they have on some of their coolers)

Arctic Cooling have the Accelero Mono Plus, which is a single 120mm PWM fan solution http://www.arctic.ac/en/p/cooling/vga/455/accelero-mono-plus.html?c=2182. The fan range is 400-1500 rpm which suggests that a 500 rpm idle speed might well be feasible. The Mono Plus costs more than the Arctic Twin Turbo II from my local hardware dealer, so the Twin Turbo might be worth serious consideration.

I don't know the 4750.
There was a short lived 4770 with a single 6-pin plug that was borderline passively cool able.
There was a 4850 with two 6-pin plugs that needed a fan.
If your card is the latter than the 7870 needs a little more power the 7850 about the same or a little less.
The Arctic coolers mentioned are compatible. Arctic also sells a new S1 Plus. With your current fan setup even that might be enough, but you might need to increase fan speed a bit.

4770 it was of course, thanks boost

To clarify, It doesn't have to use only one fan really, my wording was a bit cumbersome.

To rephrase, can i aircool the the 7870 quietly (fan speed of about 4-500 rpm for 120mm i guess), lets say money and size is no object, but i would rather not have a large modding project on my hand. I for instance have tried a MSI twin frozr 6950, that was abit too noisy (although that was a 2-slot cooler, i can fit 3 or 4-slot coolers in the case I have now), I also tried a AC cooler with the 4770, but that on the other hand is way "overpowered" it seems like i can cool a more powerhungry card than that.

Try the Arctic S1 Plus (new model is compatible with 78X0 series) with your current fan. Arctic says a fan is needed for the 7870, I think your current one should do fine. That's a 3 slot solution. The height is listed as 42mm, but that's just the plastic frame. The cooler itself is no taller than the old model at 33mm -> 33mm + 25mm (fan) = 58mm.
I'm still not a fan of Arctic's mounting mechanism, but you can't beat the price performance ratio.

So the S1-plus with 1 or 2 120mm would give the same cooling ability as lets say a Accelero Xtreme PLUS II?

Not quite, the Extreme has 5 heatpipes and has a larger fin area.
The S1's 4 heatpipes and smaller fin area should more than suffice for the 78X0 with a slow 120mm fan, but not the 79X0. The S1 has a larger fin spacing better suited for lower fan speeds and thus lower noise.

Wait a week or two for the various 7850/7870 products to hit the market and get reviewed. Hardly any are using the reference design. Might be a couple that are quiet enough for your needs without having to get an aftermarket cooler.

True, Sapphire, Asus, Gigabyte and MSI have craphic cards with very capable coolers.
The problem is the often enough the BIOS' fan control. The fans run faster than necessary as a precaution for badly ventilated cases or high ambient temperatures. You'll have to wait for a review to find out if any card has a silent fan control.
You could easily just remove the fan(s) of such a card with a good cooler attach a 120mm fan with zip ties...

Thanks for the tip, I plan too wait some weeks, both for that reason, and too see what nvidia has to offer, you never know, they might beat nvidia on power efficiency this time around.

As some people already know, the 7970 / 7950 cards have a bump in the cooler base and preexisting aftermarket coolers don't work on them.
In the reviews of the 7870 I saw something that could be a bump in the cooler base, resulting in the same problem.
However, Arctic cooling have listed several of their existing coolers (without a bump in the base) as 7870 compatible.
So I'm unsure about the compatibility of the 7870 with existing coolers. Maybe AC is wrong in their compatibility claim? Or I need glasses.

The frame that surrounds the GPU of the 78XX cards are basically the same as on earlier Radeon cards as opposed to the frame on 79XX cards. So the cooler on the 78XX cards does not, AFAICT, need any bump in the cooler base. That's why Arctic can list their coolers as compatible with 7870/7850.

Compare these:
7870 ref. cooler VS 7950 ref. cooler
7870 PCB VS 7950 PCB

I'm also looking for a new GPU as my Radeon 4850 is getting a bit slow.
The Radeon 7850 would be a logical successor but the 7870 seems to perform quite a bit faster.

The find out whether it would be possible to cool the 7870 with a low rpm fan I'm trying to compare the power consumption. However the results reported in the reviews seem to contradict each other.
Some reviews, like Anand's report the 7870 uses more power than a Radeon 6870, while other reviews, like Techpowerup claim the 7870 uses less power than a 6870.
I hope things will get clearer once the cards get in the hands of users because right now I can't make up my mind...

7850 reviews are starting to come out.