[all variants] What's the best GPU can I get?

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http://ubuntuforums.org – Hey guys, I have a Sony Vaio desktop with a Pentium 4 HT 3.00 ghz processor, 2 gb ram, and an nVidia GeForce 8500GT GPU on PCI. Though the 8500 is nice, I would rather have a card that is more suitable for gaming. Here's where my problems start. The Vaio only has PCI and AGP support-- no PCI Express. I only have $450 to spend on this. Should I just replace the mobo with a more modern one, keep the RAM and processor, and get a better GPU? I would prefer to find a more powerful graphics card for my computer so I dont have to spend an arm and a leg upgrading it. (Hardware)