Is it worth having an inlet fan? My case has ventilations holes drilled over the CPU and GPU locations. I'm just adding a exhaust fan (currently the only exhaust fan is out of the PSU and it gets pretty toasty due to the chimney effect). My CPU and GPU run reasonably cool when fragging (about 70/60 respectively) but I suspect the inside of the case is getting toasty warm. Certainly the exhaust from the PSU is hot and I wouldn't imagine much of that is from the PSU itself.

I don't want to fit a 120mm inlet fan only to find it makes the rig noisier and doesn't help.

I know some people run a postive pressure case but presumably with the vent holes this won't be possible and in fact might make things worse (might prevent the cool air being pulled in through the vents)?

I wouldn't expect there to be any significant airflow through the current vents, I would seal them up and replace them with proper inlet and exhaust fans. I use two inlet and one exhaust and it seems to work fine.

You mention the concern about noise, there is no need for fans to be noisy. A large (relatively) slow moving fan will move just as much air and won't make a sound. The loudest component on my PC are the hard drive clicks. Get a fan controller (preferably one that is software controlled although I just have a manual one), and let the computer take care of cooling when needed.

I don't think the Chimney effect takes place as you have no where near the required pressure drop at the top of your case. You just want cold air in at bottom and hot out the top. A nice clear, unobstructed path for the air to take will help greatly. I recently got a new case where the PSU is seperate (still in the case, but at the bottom) so it draws and exhausts its own air. This keeps the CPU and GPU separate in their respective air flows.

The other huge factor here is whwew you put the case. If its under a desk with a load of junk around it, so its not getting and fresh air, then its just going to circulate the air it heats up and it has no where to go.

Good points! The box is stuck behind the tv so there's not a lot of airflow, I'll measure the temperature at the inlet and see what I'm getting. My motherboard actually has a fancy fan controller built it so I'm going to use that to control the exhaust fan. I suppose blocking the vents on the side panel is worth a try- check some figures before and after.