Hello man, I need to know if you have an AGP or PCI-express video card slot. If you have a PCI video slot then i will suggest you getting the Radeon HD 4650 low profile, because I ran that card on a 220watt psu and it worked fine. However power consumption varries depending on what other hardware you have inside your computer, for example some processors consume more power than others.
There are sites on the internet that calcluate for you the amount of watts that you are going to need.
List some more system specs and I will get back to you
The Corsair 400CX will work fine. It has 380W +12V vs your current PSUs 218 +12V output. Corsair CX400W Power Supply Review
And you can see the 5670 power requirements from this Sapphire 5670 has a 400W PSU recommendation listed under the System Requirements tab.
Actually, the fan is on the bottom of the PSU.
Since your case probably has the PSU on the top you won't have the fan blocked off by the case.
If you had a PSU with a top mounted fan you'd just flip the PSU over anyway.
It's possible you might want to add another case fan, or get a more powerful case fan replacement, if you find your system running hotter than you like. But thats going to be due mostly to the new, hotter video card. Just monitor the CPU and GPU temps and they'll let you know if you need to do anything more with case ventilation.
No way to know for sure. Check your CPU and GPU temps now while gaming, and then measure again after the upgrade. With the bigger PSU 120mm fan temps might even go down. But with the more powerful hotter GPU I'd expect them to go up a bit.