$100 doesn't give you very many choices. It is enough to buy a 512MB HD 5670; the 1GB HD 5760 is a little over $100 and benchmarks have shown that extra VRAM makes very little difference in performance. It will be faster than your X1950XT; but it is less powerful than the HD 4850.
The $100 puts you in striking distance of the HD 5750, but you need to toss in round another $20 - $25 to buy one. It is nearly as fast as the HD 4850 (less than 5% difference), but it consumes less than half the power and supports DX11.
Sorry, I meant to write 9800gxt not 8800gtx in my original post. I'm not into getting that one if it is too much for my psu.
I think I can go up to $150 for something that will definitely handle hd movies and games like fallout\stalker . Is there much of a difference in getting a graphics card with 512mb than 1gb? Should I be more inclined in getting a 1gb card?
Generally speaking 1GB provide better performance than 512MB starting at around 1680 x 1050. Based on benchmarks, the HD 5670 is not fast enough to take advantage of the 512MB even at 1920x1200 resolution.
For reference purposes, the HD 5770 is nearly as fast as the older HD 4870 (within 5%), but uses about 70w compared to 120w, and it supports DX11.
The HD 5670 is about 25% more powerful than the HD 4670 and consumes less power, but is less powerful than the HD 4770.