in benchmarks you will notice a difference. however, in the real world i.e. gaming, you won't notice a difference. if you do, it'll be just a couple fps. now if you have 4870 X2 then yes, i would go pci 2.0. for a single card you're just fine..
-Desktop- Dual Monitor, Q8200 2.34Ghz, 8GB, 1TB, XFX 6850, 7 Ultimate
-ASUS UL50- SU7300, 4GB, 500GB, G210M, 7 Home Premium 64, XP Pro
-HP NC6400- Core Duo 1.86Ghz, 2.5GB, 64GB SSD, x1300, XP Pro