^^ 6800gt does not beat a x800xt - it's got a very, very similar performance to a x800xl in the real world.
Just because you have one does not make it faster!
6800 ultra ~ x800xt pe.
an x850xt can be picked up for @ £145 from ocuk and will oc to 575/600.
an x850xt pe will spank a 6800gt.
5Gb for a windows partition is really too little, unless you are very strict about deleting saved files after updating and using the minimal amout of space for system restore.
Have a good clean out, defrag, run all the anti-spyware stuff, repartition windows drive with a bit more space, put the paging file on the other physical drive.