I agree with pops above me, and on top of that I would say that your wallet needs a break from that 2405. Perhaps once the next generation of GPUs come out (and games) you may be forced to upgrade to be able to run at its native res, but for now I'd say deal with merely High settings and not worry about ultra high with all features enabled, because really, your enjoyment of the game will improve a fraction of what it'll cost you.
That being said, however, if you DO get the x1900xtx, you can feel free to hook me up because my 9800 Pro could REALLY use an upgrade (Oblivion on 640x480 on Medium settings is breaking my heart).