Just don't make it biased and pro-nvidia. If you do, you'll be getting emails for the rest of your life from every ati loving inebriate in cyberspace.
Make sure you do benchmark comparisons. Benchmarks mean nothing to most people unless it's pitted against another recent graphics card (TI line of Geforce 4).
Make sure you test stability on Direct3d games. A lot of users have been having lockups with them.
Don't go overboard on 3dmark because no one cares anymore.
Try some FRAPS averages on new games like Mafia and NOLF 2. Graphics are finally starting to get better, and there are some system-straining Direct3D games being released that struggle for high framerates.
Just remember, don't make it anti-ati. You will get so much **** for that you'll wonder why you even wanted to write the review in the first place.
2.66 P4 @ 3.0 Ghz / Geforce 6800 GT @ 410/1080/ 768 mb Samsung DDR333 @ 374 mhz / Abit IS7 / XP Pro / NEC 1760NX LCD