Also, if you wanted to compare performance then you have to look at review and benchmark of both cards on the same website. However, the website must have a review or benchmark for both cards. Comparing video cards by using 2 different websites is not always a good idea because sometimes 2 different websites or 2 different testers use different Test Setup like different motherboard or different CPU while testing the video card. For example, let's say that Bittech.com might use Gigabyte GA-P35-DS4 with a Core 2 duo CPU to test Geforce 7800 GTX and Techpowerup.com might use ASUS P6T-Deluxe with Core i7 Extreme 650 to test Geforce 9800 GT so this does not make a good valid comparison because both ASUS and Gigabyte motherboard and 2 different CPUs might give a different test result of performance. Sometimes you also have to look at the date of when it was tested and what driver version they use when they made the benchmark performance because sometimes, different driver version could give a different test result of performance. To check the date, just simply check when the review/benchmark was posted on the website by the author or check it on the benchmark page (if it says) and the test/system setup page or the benchmark page might tell you the driver version.
This is the kind of thing I've been doing for a long long time but it is not easy because sometimes the same website does not have review/benchmark of 2 cards that I wanted to compare and sometimes they use different test setup.