I have just put a Kraken X31 (120mm Radiator) on an R9 290 using the Kraken G10, works fine naked with no VRM 1/2 cooling or RAM cooling, at stock, for games. So a 120mm/140mm radiator is perfectly fine, a 240/280 is going to make a second card installation later on really difficult in any case (assuming you have an AIO cooling on your CPU too).
Don't be fooled by reviews, the fitting process difficult, more so than it needs to be and this is mostly due to the cheap design and parts of the Kraken G10 bracket, it's no where near the kind of mounting quality you get with your included CPU mounting equipment. So I probably wouldn't recommend it to newbies, an aircooling solution might be the way to go if you have a reference model you want to cool as they are generally pretty easy to fit.
I the X31 at 80% fan speed and pump on full (3500rpm), GPU tops out at 45 and VRM 1/2 both sit at or below 60 with the card at stock speeds, in games.
You will notice your VRM temps climb to like 80/90 or so in Furmark or something, just like your power consumption does and thus the extra VRM heat, although it is not really representative of any actual gaming and its not really out of spec for the VRM modules either.
If you overclock and push more power into the GPU or in particular raise the voltage, this is where the VRMs will just pop right up to 90-100+ in gaming and you could probably do to cool them, but they are apparently rated to 125, although anything higher than 90 is probably worth seeing to.
I will note that my VRM modules do seem to run cooler than most and I think is due to the fact i am running my card in reverse orientation due to my case layout (so it is technically up side down). I would guess that 80% of the heat rises right up into the fan which dissipates the heat better then a standard orientation where probably 40% of the heat is rising through the back of the PCB (as you have no heatsinks) where there is no direct air cooling. If you popped a fan pushing air directly behind the GPU VRM, I would think you would see temps fall by 10 or more, which might put you at a level that's more acceptable (without spending more money). This is moreso validated by some people adding thermal pads and heatsinks to the back side of stock cards and seeing temps fall as much as 20 on the VRM on stock cooling, which can also drop GPU temp, examining the reference cooler you can see why this would be the case with hot VRM air being the first thing drawn across the GPU.
If you are going to benchmark quite a bit (for some reason), mine bitcoins or overclock with increased power and voltage I would totally recommend buying the Gelid Enhancement Kit for R9 290 this includes perfect fitting heatsinks for VRM 1 and 2 and I can confirm they fit under the G10 bracket. They will drop your OC or high stress VRM temps to around 75 max (lower on a stock card), which is a lot less than some branded aftermarket cooling solutions that can run up to 95 (which is why we can also deduce these temps are totally within specification of the parts).
Having said that the Gelid VRM heatsink kit is available almost nowhere, as everyone obviously figured that out and is buying them. You can pickup some on eBay, but it's going to cost you about double what it probably should and it's a pretty cheap looking aluminium part. I would advise getting some proper thermal pads for the push pin VRM sink (Phobya) and some proper thermal tape (Sekisui) for fixing your smaller VRM sink and any RAM sinks you may decide to add. The materials in the kits are garbage and so is the thermal paste that comes preapplied to your choice of boxed liquid cooler, I added pretty cheap Arctic Cooling MX4 and saw a drop of just under 10% in temps, this is because your GPU has no integrated IHS like a CPU, so the contact area is going to be more important, much like a de-lidded CPU.
If you are just gaming and not heavily overclocking with more voltage or immense amounts of additional power, I can say it is totally fine to run it naked. I would just suggest buying the Gelid kit of you are thinking of overclocking a bunch, for sure. However first drop a 120mm fan behind the card if you case allows for it and see how far you get before adding more expense.
Hope this helps.