From Dell's web site:
Microprocessor type
Dimension Lxxxr
Intel® Pentium® III microprocessor that runs at 500, 550, or 600 MHz internally and 100 MHz externally, or a Pentium III processor that runs at 600, 667, 733, 800, 866, or 933 MHz, or 1 GHz internally and 133 MHz externally, or 1.1 GHz internally and 100 MHz externally
System chip set
Dimension Lxxxr and Lxxxcx
Intel 810e chip set
System clock
Dimension Lxxxr
100 or 133 MHz (matches external processor speed)
So it sounds like if you don't mind upgrading the RAM, which is dirt cheap right now, you can go with any PIII.
-KeithP