The upgrade will increase your frames for the game, you can see here an example:http://www.geforce.com/Hardware/GPUs/geforce-gts-450/performance/Witcher-2-The-Assassins-of-Kings-NVIDIA-GeForce-GTS-450-OPS
the settings you will be able to play witcher 2 with acceptable fps... 30+
but if you are planing to play on high graphics you will need a better card than this one, but if that is already ok to you, go for it.
you can check for a 550ti, close price, and better performance.
About the store-bought, you must see how the warranty there works, some stores works with warranty in Each Component, and others if you open the case you will void it...
make sure your psu is from a good brand, this card requires 400W some 500w blow up at 350w, so you must check if your psu brand is trustable and will handle your video card without problems, if it is possible give the model of your psu.
probably your psu will have at least one pci 6 pin connector in the last case you can use molex to pci adapters.