512mb 6800ultra doesn't matter at all because games don't use up to even 256mb
also, if i were you, i'd just wait until the summer when nVidia's new NV70 core and ATi's new R520 core gfx cards come out
they will start with 24 pipelines and eventually they'll sell ones with 32 pipelines
those will be some kickass cards; and even if they are really expensive (i'm pretty sure you can afford them though), the 6800gt prices will drop