1GB of memory on a graphics card definately has potential (it's not useless) but the problem is the R600 GPU is not powerful enough to utilize it, just as it is unable to take advantage of the bandwidth offered by the 512-Bit bus. When the first 512MB cards came along, such as the X800XL 512MB, everyone thought 512MB was useless, far too much memory - however, when faster GPUs such as the X1800XT came out, 512 pulled way ahead of 256. When faster cards, such as R700 come out, then we'll see 1GB of graphics memory be utilized properly.
You would have been better off going with an HD 3870 for cheaper or an 8800GT for the same price. The 2900 Pro isn't bad if you can overclock to XT speeds, but an HD 3870 would still be slightly faster and an 8800GT much faster. I'm sure you'll be happy with performance though - the HD 2900XT is still no slouch.