the amount of onboard memory a graphics card boasts isn't overly important. You'll hear people go on about how much better the 512Mb version of the 6800 will be than the 128 and 256 versions. The truth is, there is absolutely NO games that will utilise 512Mb. In fact, there are probably few that would utilise 256Mb. Maybe in a few years time, 256Mb will be the norm, but for now it is totally unneeded. The fact is, the 6800 will still hurt video cards like the 9800 Pro even if it only has 128Mb.
If your worried about it just set your vid card up to use 100 megs or so of your system ram (if you have enough).