First off the gts 250 is a superior card to the 4650... you will see an increase of about 20fps (frames per second) going from a 4650 to a gts 250.
I don't know if opening your case voids your warranty or not...
One thing you should look into is according to the website the power supply requirements for the gts 250 need to be above 475W, does you power supply meet this requirement (2 of the 3 gts 250 cards on the website state this)
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below are some of my most commonly asked questions as these will help me better assist you:
whats your budget?
what are you using your computer for?
desktop or laptop?
