There is a wide variety of GPUs (Graphic Processing Units AKA Graphic Cards) available in the market today that match your needs according to your budget and performance expectations, namely two to choose from; http://www.nvidia.com nVidia and http://www.ati.com ATI. GPUs offered vary based on their capabilities with basic card of 128MB ranging upto 1GB easily. You can look for one that matches best your needs and budget and read it's reviews from a variety of websites, popularly http://www.tomshardware.com Toms-Hardware and http://www.anandtech.com AnandTech and for getting an idea on pricing, do check out [NewEgg|www.newegg.com and http://www.ebay.com eBay.
Though this is entirely your prerogative to go for anything that best suits your needs but then you should also ensure that the card you are ordering is in compliance with your PC.
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