That won't make a difference. Oblivion only uses just under 2GB when it is running, no matter how much RAM you have. You can use a patch utility to double the amount of RAM Oblivion can use but that's still under 4GB.
Most programs are like this by the way.
I tried the 4gb patch a while ago, and it has absolutely no effect on the Oblivion's performance. At least not for me.
A simple answer is that DDR3 memory is dirt cheap right now. I wish DDR2 was similarly inexpensive. I'd love to go from 4 GB to 8 GB on my secondary rig (which has its own Oblivion install, used for mod testing). That one runs Windows Vista 64, which is more a resource hug than Win7. I can't justify an OS upgrade. Truth be told, now that Vista has matured I like it as well as Win7 for what I use it for...except for the extra resource footprint.
I went from Windows XP, to Vista, to Windows 7. Of them all I prefer Windows 7, with XP coming in second. I strongly recommend you scraqe up the money for Win7. It is worth the improvements.
It does help to have a good CPU too and not just RAM. I run Win7 64 3Ghz and as I said I see no need for more RAM right now. But 12 RAM.. damn!
What Decrepit said about the Ram. It did not cost me all that much. Besides, my box will take up to 24 gig, so I am only using half its potential. But I am happy with how it runs on only 12, so I'm sticking with that. Everything runs great except for Oblivion, and I do not blame my PC for the latter's issues. It has done the same thing on 3 different computers I have owned.
I guess I'm strange (what else in new?). I don't really care what my framerates are. If the game feels right, I'm happy. If it doesn't feel right, I do something about it. I stop doing something about it when it feels right again. Over the years I have adopted a moronically simple attitude toward technical matters, lol.
I do not usually bother checking my framerate. I only look when a game is running poorly, and I am trying to figure out why. That is what exasperates me so much about Oblivion. I have ten times the computer I should need to run it, and still it runs like crap.