I don't have a definitive answer for whether you'll be able to play at settings that are much higher than what you're currently using or not. If you're content with playing at 1280x720 then I'm sure you can successfully hold out until you can afford a video card upgrade with your new system. You could always just lower your background FPS to like 10 or 15 and free up almost 80 wasted FPS that your slaves don't need which will give your GPU some breathing room for possibly higher settings on your main.

What is 1go and 2go? I'm assuming that I should be reading that as GB for gigabyte. Video memory isn't the most important factor because if your card doesn't have the horsepower to handle filling it up, then your performance is still going to suffer. Here's a GT 640 with 2GB of RAM, but you won't be able to 5-box with it because it doesn't have the horsepower. Memory Bandwidth and Texture Fill Rate are probably the most important specs on a GPU and you'll need to look at reputable hardware sites to see where exactly your current card falls versus a 560 Ti in a benchmark. The 660 Ti also just came out which costs about $60 more than the 560 Ti, has more video RAM, and is just a faster card overall.