I don't believe Win98SE will have anything to do with hyperthreading, but this shouldn't be an issue. It should just run like a system without HT, under Win98. As for finding drivers for all your hardware, that's going to be another story. You almost certainly won't be able to get them from Dell, and will have to look for them at the manufacturer sites for the individual hardware.
You're also going to have to format the drive completely, and start fresh, as you can't add Win98 after XP is installed - it needs to go on first, to configure a properly functioning dual-boot scenario.