I was looking into the graphics and if you are playing on 1 monitor/ decent high resolution- 1 graphics is fine, and in comparison - WITHOUT MESSING WITH OVERCLOCKING- the HD3870 beats even the 8800 Ultra(except the 256-bit)- and you can get 2 HD3870s for the price of an Ultra- crossfire them together and make up any differences.
320Stream Proc, 800MHz coreclock, 2400 Memory clock, 512MB, 256-Bit GDDR4
and as for gameplay differences- I highly doubt you will notice any.