Are you overclocked now? If not then you'll need to replace the stock heatsink with an aftermarket one. If you are overclocked, you're fine.
I don't foresee any bottleneck in adding a 560ti in SLI, looks to be a sound step IMO
what do you need the system for? I assume that you are asking if you have room to overclock, and the answer is: limitedly. It appears that you are using the rather poor stock cooler. that will need an upgrade if you want to overclock. Whether you can get an aftermarket cooler and which depends on whether you are using a name-brand manufactured computer. you need some room in your case.
You missed a deal on the release of the zalman lq series, but you can still get good price/performance from the hyper 212 evo or the noctua dh 14, which are air coolers.
You could add an identical gpu, but that would require the gpu to be sli ready and you would need an sli bridge. The drivers have not been fully developed for sli, but there is an increase in performance. Keep in mind that it is not doubling the graphics performance because there is a bit of latency over the bridge and the drivers are not complete.
sorry forgot to mention i have the hyper 212 evo installed. and why is the room to OC limited?
you max out around 60C, and the max temp it should be able take with no extra damage is 70C. That gives about 10C overhead. you could overclock easily to 4.2Ghz, but you never want to go over 4.4Ghz with that cooler. I have one in push/pull, and 4.2 is what I would recommend for initial testing. if it's still below 70C, add 0.1 and try again. Change voltage accordingly.