Hi guys, I was wondering if I could ask for some help?

I'm running a pretty old system, 2008, and it's been running Oblivion fine-ish since forever. Recently my Sapphire Radeon 4870 made its last death-gurgle, so I replaced it with as like-for-like as I could, an XFX Radeon 4780 x2 I managed to get for ￡20 (I can't afford a new graphics card right now, let alone a new system). I modded the PCI-E connector from 6 to 8 pin myself, updated all my drivers with Driver Reviver. Everything's fine, no problems with display, Skyrim runs just fine with the new card.

So I open up the Oblivion start menu so the game can detect any changes in hardware settings. I get a popup telling me my hardware cannot be detected, and graphics quality has been defaulted to medium. Ok, I think, it's an old graphics card. Whatever. Oblivion worked fine with the Sapphire and Skyrim doesn't seem to have a problem with the XFX.

But now Oblivion crashes on startup - not even CTD, but a whole system reboot, apparently so nasty it asks if I want to run windows in safe mode. Weirdly enough, I seem to get varying results by messing with the resolution - in some resolutions I can even get as far as the Oblivion menu and start loading a game... but I can never actually get to the game before it crashes.

Can anyone tell me what I'm doing wrong?

The 4870 x2 is a multi-GPU (it's basically two 4870's bolted together) card that is old enough that it might have a Crossfire profile for Oblivion but not Skyrim. So when Oblivion starts, it starts in multi-GPU mode and the card starts drawing power for the two GPU cores on the card whereas it only draws for one when running Skyrim in single-GPU mode. This increased power draw can cause a brown-out in your system, unexpectedly resetting the computer after which Windows proposes starting in Safe Mode, as it does after any unexpected power loss event.

Check that your power supply can provide the necessary power (about 300W i.e ~2x the 150W of the old 4870 you had, + whatever the rest of your system draws, so ~500-600W in total) or go the ATI/AMD GPU control panel and override the setting for Oblivion to disable Crossfire.