A "usable" video card may range in cost from the $80 price area your X300 is in, all the way to $500 or so (not sure) for the current king of the mountain X1900XT VGA. Personally, I would suggest going ahead to bite the bullet and get a card that will last for awhile, at least mid-range, around $200. For AGP, the 6800GS is sitting on the spot of maximum capability/ speed per dollar that the 6600 GT held for so long, and the Radeon 9600 Pro had fit into before the 6600 GT came along.
The Geforce 6600 and X700 will cover the basics (and perhaps a little beyond that) for Oblivion, but I would rate those two as marginal for the purpose, and too likely to need still another upgrade within perhaps a year, as a result of being minimally acceptable today. I'd move up the chain a notch,
at least, to X700 Pro, and 6600GT.
I'm not doing much with PCI-e so far, but I believe that there is a 6800 GS for that interface as well. The various X800's are slightly less capable, but also slightly less expensive than a 6800 GS; you can find models in both AGP and PCI-e that will do the job now, and still be useful a couple of years from now.
Incidentally, I visited that evaluation site when the 3-20 release date first became official, and used a PC running Windows 2000 Pro. That system failed the OS requirement at the time, but I installed Obivion anyway (however, in the interim, Bethesda had amended their official requirements at some point late in February, and included W2K as acceptable -- I'm sure they didn't change any code before changing the published requirement).
[ 03-27-2006, 07:57 AM: Message edited by: The Kiwi ]