Faster ram > more ram to an extent. Its limited by the resolution that you are going for. The higher the resolution (basically 1900x1200 upwards at full aa/af) makes more use of the extra memory, but the flip side is that the rest of the system might not actually be able to cope with all that anyway so you will be capped at a certain performance level and the extra ram would be wasted.
the only real difference between the 4850 and 4870 is a slightly faster clocked GPU and gddr5 instead of gddr3 which offers more memory bandwidth.
stop with the ATI hate, i've yet to have any issues with them at any point over 4 different ATI cards.
My opinion on what graphics card to go for is really to look through benchmarks and choose what is the best value (not outright performance) for the particular game you will be playing most as the performance varies so much between nvidia/ati on different titles because of extra optimisation in drivers. (Quakewars is fairly nvidia dominated as ID kiss the ass of nvidia)
And for the love of god, put some context on "future proof" as it doesn't really make any sense in the pc world due to fairly dramatic changes every 6 months with hardware.