my recomendation... put fans on the side of the case, it looks like you have 2 places where it could go, high and low.
i would recomend a 120mm in the lower side panel position blowing inward. this will blow cool air directly onto your 7800. Also, put some AS5 on the 7800. i took the hsf off my 7900 and the grease was all crusty and hard... it was horrible. my gpu droped serveral degrees after i changed the paste.
the air blowing in from the front is probably blowing across your hd's and picking up several degrees C in the process. this is great for you hd's but bad for heat transfer in the back of the case. I cant really see where the front fan is positioned... regaurdless... put the hd's infront of it if you can!
Put another fan, probably 80mm so you can still use that tube, blowing inward in the upper, side panel. this will blow cool air directly onto your cpu.
If you've got a 120mm blowing out of the back, you should be money.
If you add these 2 fans, a 120mm and a 80mm (not sure if the side tube is compatable w/ the 80, might be 92mm or something), to the side panel of your case, you should see at least about 5*C temp drop. I would recommend getting fan dampners for them as well as they may vibrate against the side of your case.
well, thats a heat transfer engineer's opinion... i'm sure some 12 year old will disagree with me but whatever.
because i like seeing myself in type....
120 mm in lower side case grill blowing in.... 80 mm in upper side case grill blowing in... 120 blowing out back
Artic silver 5 on all your hsf/prosessor interfaces
no worries mon
****techinical heat transfer jargon****
remember that heat transfer is driven solely by temperature gradient (AKA temperature difference) if you dont see a difference now, it does not mean you wont notice a difference once you start overclocking.
Say your processor is running at 50*C and the ambient is 25*C. the gradient is 25*C. when you blow more air across your parts, the temperature of the processor drops slightly and the air heats up, both lowering your gradient therebye lowering your heat transfer.
you start OC'ing and your processor goes up to 75*C. the gradient is now 50*C. you will expect to see 2x the heat transfer. this means that, all things being equal, you will need 2x more air moving through your case to maintain the heat transfer.
the reason for needing more air is this. If you have slow moving air in your case, the temperature of the ambient will rise quickly. say from 25 to 40*C. in the OC'ing example your temperature gradient goes from 50*C to 35*C. this means that your heat transfer has reduced by 30%. this is significant because when you are overclocking you are producing more heat than normal and the problem starts compounding itself.
In short, the point is to have constant, low temperature, air blowing briskly across your parts. Not because it will drop your temperature drasticly, but because it will keep it at an acceptable temp as your processor puts out more and more heat!
This has been a public broadcast of your local heat transfer specialist. Thank you.