I have converted my video to sequence of tga files as I have read the work should be quicker and better as I would like to cut the video. However the framerate I got is somewhere about 10fps instead of 30fps in preview. I have tried to create proxy - 50% but it had no effect on the speed, turning on and of AV sync - nothing. I have set prefetch frames to 500 and RAM limit to 4GB as I have only 8GB. I have older graphics 9600GT, but I think it should be OK, right? CUDA is on. What am I doing wrong?
You should be able to proxy, but you haven't given quite enough information to make offering a viable answer entirely clear.
Let's assume that the TGAs are of very large dimension, say 4k. A 50% proxy would only put the resolution at 2k, which is still likely too taxing for your system if I had to wager.
Best advice is to start at the lowest possible proxy resolution and build up. If proxies are failing you, then generate your own proxies using a lightweight format such as a JPG, and keep the dimensions small.
Also bear in mind that the VSE isn't much of a compositor, and as such, you can seriously tax your realtime playback by stacking several semi-transparent strips. Even if proxied, the layers will all be composed on your CPU and will likely drag your system's performance well down. The tip that gandalf3 linked to is also relevant and worth respecting.