The antec 300 has the PSU located rather close to the video card and both seem to be competing for air.

I'm wondering if it's good idea to get a "cross blow" PSU , the older layout where the fan would blow through the psu horizontally. I'm not seeing many of these on the market anymore... any popular ones ~550W+?

I haven`t seen any higher end cross-flow power supplies for a while though some budget models like the FSP 350GLC or Antec Earthwatts 380D have used 80mm fans until recently. You might be able to find something like that in stock. Such units are usually rated for no more than 400 watts and efficiency won`t be the best but they should do the job.

It`s a shame the 300 doesn`t have an intake vent on the bottom for the psu. I would be tempted to cut one out myself which is what I did when I had to face a similar situation.

Antec Signature?

Reasonably efficient and very high quality, if slightly older, design. I've got one in my PC and it's good, although I have swapped the fan. The stock one runs about 700rpm minimum and is reasonably quiet.
PSU casing is quite long and has modular cables.
Seb

Antec is a good option, another coould be the kingwin srtiker 500W passive, being fanless it shouldn't interefere with the gpu.