I compared the temperature readings of my PC with the side panel on and off... they were the same even though there was only a single 120mm fan in the whole case. One of the temperature reading actually decreased when i put the case back on. The room was also AC cooled so its pretty cold!
So if i add another fan, there wont be any difference right? THANKS! ( BTW i have a cpu cooler also )
Well, that all depends. Air flow is tricky, as you probably thought that removing the side panel would lower temperatures and it actually raised them slightly.
Ideally you should have intake fans in the front (lower right with the side panel removed) and exhaust fans in the rear upper (upper left or blowhole top). My guess is with a single 120mm fan, that is an exhaust. I've bought many cases that didn't have an intake fan in front.
Adding an intake fan will increase air circulation, which is what we really want. Good airflow through the case will keep pockets of hot air from forming around the heatsink and motherboard.
On a fairly standard pc, you won't need a lot of fans running which will increase noise. Add a hot graphics card or a high wattage cpu and heat can build up quickly.
In other words, unless we know more about your system it would be hard to answer how much difference adding a fan would make. It doesn't sound like this is an urgent situation though.
Intake fans would surely help.. Thugh, a front intake fan will be more beneficial compared to a side intake fan.. My soon to be replaced gaming rig has an E6750 overclocked to 3.2 GHz and a radeon HD 4870 along with two hard drives all housed in an Antec TX 1050B which has a single rear 120mm exhaust fan.. No heating issues till date and my room does not even has any air conditioned cooling.. I guess the 5770 produces even less heat than the 4870..
Looks like temps are not an issue. If you wanted to add a fan it wouldn't hurt, a front intake would be best as emperus suggests. You have a fairly simple setup so it doesn't appear to be necessary however.
Installation of front intake fans can be troublesome as the front panel often needs to be removed, and you may need to remove or at least shift some hard drives too. To me, it's a worthwhile project but I enjoy tinkering.
You could add a side panel fan, as the one on your case would supply fresh air right over the CPU.
Just a pointer regarding your video card.. Upgrading to a 5770 is not going to give you any performance benefits over your current 4890.. It'll just bring features like directx 11, eyefinity and little lower power consumption & heat levels.. For higher performance, you need to start looking from the HD 5850 from the ATI camp or the GTX 470 from the Nvidia camp..