I just upgraded from a 4850 1 GB to a 6870 2 GB and I haven't seen a huge improvements. Maybe I'm expecting too much out of the card or maybe there's something bottlenecking it, hopefully someone here can help me out.
here's my specs
Motherboard - Pegatron IPIEL-LA3
CPU - Intel Core Quad Q9300 2.50 Ghz
PSU - Delta DPS 460 Watt
RAM - 6 GB DDR3
GPU - XFX Radeon 6870 2 GB (OC'ed to 950/1100)
I don't think the video card is bottlenecking. The 6800 video cards aren't top of the line but it's not like your pairing a I7 extreme with a Radeon 5770.