Your Computer has a Slot one PIII with a 100FSB. It is capable of taking a slot 1 1GHz 100FSB PIII. You CAN still find 1GHz Slot 1 100FSB processors around , but they're getting rare. Googlegear.com has them for $209.
Googlegear link to Slot 1 PIII 1GHz 100FSB Processor
Ignore the "only tested in server" warning on the box. They work in Dell XPS-R and T computers, I have one in mine, and hundreds on the user boards at Dell have done it too.
You can get advice on this VERY POPULAR and always sucessful upgrade here at the Delltalk message boards
http://delltalk.us.dell.com/
And you can find detailed info on Dell processor upgrades here:
Dell Dimension Processor Upgrade FAQ
BTW, there are no FC-PGA 1GHz 100FSB processors. The highest socket PIII with 100FSB is the 850MHz.
Go with the Slot 1 PIII 1GHz 100FSB. It's as easy as popping the old one out, and putting the new one in. Just remember to plug the fan on the heatsink of the new processor onto the motherboard. Also, you need to make sure you have Dell BIOS A09 or later (You most likely do). If not, go to Dell and d/l the latest BIOS revision and flash it.