Hi first you might like to read this benchmark test on a range of cards similar to those you're considering :
http://www.tomshardware.com/graphic/20050404/index.html
At the same site - check out Motherboards as PCI Express is only something available on the latest Intel (after P4) and Athlon 64 chipsets - you'd need a new mobo to use it.
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Some other decent sites with forums here :
http://www.hardocp.com
http://www.sharkyextreme.com
http://www.techimo.com
Ask those guys what else you might do to beef up your system - as merely putting in a new vid. card will possibly only means there's a bottleneck elsewhere in your system - ie slow HD, insufficient ram, weak power supply etc.
Should you need to get more info on your system spec - grab Belarc Advisor here :
http://www.belarc.com
It'll give you a full profile of your pc setup - h/ware and s/ware.
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Lastly in case you want it fior the future - this site offers tweaked graphics drivers (and have a help forum / faq) :
http://www.guru3d.com