If you're only on a limited budget, then I don't see the point in getting a new GPU. Even if your current GPU is playing up, stick with it for now and when you get more money save up for a good GPU. Then it will be actually worth it and you won't be disappointed (If you get just a regular not very good GPU now, then you probably won't be impressed with its performance). I waited years for a good computer, i've had it for about 8+ months now and I'm still really impressed by its performance! (I have a GeForce GTX 650 Ti GPU)