While it is entirely possible that you could replace all of your hardware, then boot your existing primary harddrive to a perfectly functioning OS, I wouldn't want to risk it. Right now, via its registry and other means, your OS knows every piece of hardwired hardware inside your box and has applied drivers to each, some of the drivers native from your Windows install while other drivers were acquired from packed-in CDs or downloads. Replace all of that hardware and your OS only has the existing drivers to go on. Admittedly, I guess you could eliminate your video and sound drivers before doing the transplant, but the motherboard? I'm thinking no.

If it were me -- and it has been me a few times now -- I'd back up everything that's valuable (including all of those downloaded drivers) onto CDs or some other relatively stable media, expecting to greet the newly installed hardware on first boot with a Windows install disk and an order to format the existing boot partition. It will be a pain to wait out the install and then to reinstall all of your existing apps and games, but in the end, your registry won't be cluttered with misdirected drivers.

But do you really have to go so far? You've got a speedy Athlon, something that is still a ways from being obsolete. In the long run, it might be a bit of a bottleneck, but I'm thinking that your better option is to replace the video card and add more to your existing RAM. You've not said how much RAM you're running, but maxing it out will make a lot of difference.

About the video card, your card is about one grade below mine. I know for a fact that my FX5900 turns Oblivion into a filmstrip. I was able to make it through the initial tutorial level, as the field of vision was manageable and the textures were uniform, but once I stepped out into the city, it started to creep. And the forest? Ungodly slow. (And yet, Half-Life 2 is entirely manageable. Odd.)

So shop around, get more RAM, and get a new video card (Newegg always has excellent deals and dependable user ratings) that can handle the HDR, the shaders and all of the other goodness. Personally, I'm considering an upgrade to the Nvidia 7600GT. It's not the latest and greatest, but it has been tested and made a decent showing with Oblivion.

Hey, thanks for the reply...
RAM - I'm running with 1 gig, and I can expand to 3 total. I'm looking at getting a 6200 258mg (AGP, preferably) which is cheaper than RAM, and could solve a lot of the lag. The cards that are higher than that are currently out of my price range, so I'll have to make do. I'm sure that the differences between HL2 and Oblivion are the poly's and all those branches on the trees and the grass. Grr.
Yeah, the MB swapout would be the _last_ thing I'd do, as I was fairly sure that I'd have to reinstall Win, but wasn't positive.