lol i totally forgot about the my system thing :s
the x850 xt/pe was pcie as i stated in the post so obviously of no use.
however as i said, and now it has been highlighted that oblivion uses sm3.0; i would advise the 6800gt
you should prob be able to find some nice cheap 2nd hand deals going on, and as mike_w said it should also overclock to ultra speeds
for best bang for buck from that group the gt should do it for you. especially as it has the sm3.0 support.
as for psu, now thats a hard one as dells do funny things with their systems. check the ratings on the rails (esp the 12v one) and report back.
6800gts can work with the shuttles 200/300w psus, but they are particularly strong psus for their size. you could buy a fully fledged psu now and keep it for when you upgrade; there are loads of great brand psus so just check out for reviews, modular is handy as well. however just check the space situation in the dell and whether it is indeed a standar psu :s
Ian