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.