I had a similar setup with a Phenom II, dual-booting W98SE/WXP, and an ASRock A770DE+ with two PCIe x16 slots. For W98SE, I had a GeForce 7600 (or a Radeon X700 with a native W98 driver) in the first PCIe slot. For XP, there was a Radeon HD3870 in the second slot. In the OS, the second card was always disabled in Device Manager (the 3870 in W98SE and the 7600 in WXP), and the outputs from both were routed via a VGA connector for W98SE and a DVI connector for WXP to a monitor with both types of inputs.
The system would boot primarily into Windows 98 SE; booting into Windows XP would proceed all the way to the desktop (without a POST screen), since the card in the first PCIe slot was always initialized first.

It depends on whether the bios (probably the uefi firmware in your case since it's an i7) has the option for selecting which gpu is the primary gpu. If it does, then you can select the "98/xp gpu" as the primary gpu in the bios and boot into the legacy os. You don't strictly need to select the card as the primary card in the bios but i believe you won't see anything on the screen until the os boots and the drivers load up if the card is not selected as the primary card. Also keep in mind older gpus might have issues displaying the uefi firmware.