It's not preference, really. When you weigh the 7 series vs the x18/9 series, the 7 series cards are much cheaper, and give much more bang for your buck. if you're talking about the 8 series, that's because they're pretty much the best that money can buy at the moment.
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AMD X2 3600+@3.00|2GB Crucial Ballistix Tracer PC2-8500|evga 7900 gs (750/1780) Volt-moded|ASUS M2N32-SLI Deluxe|2 x NEC Multisync 90GX2 19"