Uhm, I can probably skip the HD upgrade altogether, at least for now, as I have an extra 250GB drive. So I can probably afford a 200€ GPU, maybe even 250€, but if it's gonna cost me more than 400-500€ altogether, I'd prefer to get a new powerful laptop for around 1000€.
EDIT: A friend of mine says I should also change my mobo and CPU. My mobo definitely isn't the best (only supports 4x 1GB RAM, only has 1 PCIe slot, doesn't support OCing...), so he might be right. What do you think? This is the manual for my mobo: http://scr.bi/ik4bBm