By far the best option I have found for easy Windows 98 retro gaming builds are Dell PCs with a Pentium 4. The Pentium 4 is the fastest CPU type officially supported by Windows 98 and you can easily setup a dual-boot Windows 98 and XP machine on the right models.
I personally prefer Dell's older PCI-only models like the Dimension 2350 rather than those with AGP because they changed how their BIOS worked and you end up getting hardware conflicts with the newer models that there is no option for you to fix. For PCI these systems really hit the sweet spot though as you can always exit Windows to DOS, use Windows 98 with 6 year newer hardware (which makes it ZIP along) and use Windows XP (released in 2001) with CPUs released up to 2006 (making it a decent machine for many XP titles as well).
A good 98/XP machine will allow you to run pretty much everything that doesn't require an older machine (DOS titles that require a Turbo button) or out specced titles that you can still run fine on the current versions of Windows.
As far as hardware I would recommend researching how to remove the heatsink and giving the CPU some fresh thermal paste but be careful as the pins are attached to the CPU and you can occasionally bend them while removing it. I usually disable onboard everything and install a Soundblaster Live! Soundcard for a decent level of compatibility, then I vary the video cards by whatever I am wanting to run. The best PCI video cards is the GeForce FX 5500 but a 5200 would run mostly everything you'd want to. And Windows 98 does have USB support when you install it (which makes transferring games or anything else a breeze via flash drives): https://www.philscomputerlab.com/windows-98-usb-storage-driver.html
Another consideration is that sound card. Some DOS titles are extremely temperamental with PCI cards and you may have to buy something like the Yamaha YMF724 series to stop lockups. IndyCar Racing II is really bad for this, and it's worth considering really searching to try to find a board with an ISA slot and getting an older soundcard for that slot type if ICR2 is your reason for doing this to try to run it on a Rendition GPU. A Pentium II 200Mhz or so might be a decent level of system for that, and is more likely to have an ISA slot... That's the only sim title that I'd go lower than a Pentium 4 though...
I personally prefer Dell's older PCI-only models like the Dimension 2350 rather than those with AGP because they changed how their BIOS worked and you end up getting hardware conflicts with the newer models that there is no option for you to fix. For PCI these systems really hit the sweet spot though as you can always exit Windows to DOS, use Windows 98 with 6 year newer hardware (which makes it ZIP along) and use Windows XP (released in 2001) with CPUs released up to 2006 (making it a decent machine for many XP titles as well).
A good 98/XP machine will allow you to run pretty much everything that doesn't require an older machine (DOS titles that require a Turbo button) or out specced titles that you can still run fine on the current versions of Windows.
As far as hardware I would recommend researching how to remove the heatsink and giving the CPU some fresh thermal paste but be careful as the pins are attached to the CPU and you can occasionally bend them while removing it. I usually disable onboard everything and install a Soundblaster Live! Soundcard for a decent level of compatibility, then I vary the video cards by whatever I am wanting to run. The best PCI video cards is the GeForce FX 5500 but a 5200 would run mostly everything you'd want to. And Windows 98 does have USB support when you install it (which makes transferring games or anything else a breeze via flash drives): https://www.philscomputerlab.com/windows-98-usb-storage-driver.html
Another consideration is that sound card. Some DOS titles are extremely temperamental with PCI cards and you may have to buy something like the Yamaha YMF724 series to stop lockups. IndyCar Racing II is really bad for this, and it's worth considering really searching to try to find a board with an ISA slot and getting an older soundcard for that slot type if ICR2 is your reason for doing this to try to run it on a Rendition GPU. A Pentium II 200Mhz or so might be a decent level of system for that, and is more likely to have an ISA slot... That's the only sim title that I'd go lower than a Pentium 4 though...