No, the XPS-R was a 440BX chipset based PII system. The XPS-T was a 440BX based PIII system and allowed you up to 800 MHz, but all were 100fsb. The XPS-B, 4100, and L series DID offer 133 FSB CPUs. They were 820, 815, and 810 chipsets respectively.
Trying to change the CPU speed higher in the BIOS wouldn't really help since the CPUs were multiplier locked anyways so the CPU would just ignore it. You would have to up the FSB to overclock, just like all other Intel systems. SoftFSB MIGHT work, but I'm not sure.
In short, no, you can't overclock a Dell. Or a Compaq or a Gateway or an HP or a Micron either for that matter. Really makes sense if you think about it. A) if you want a faster computer from them, you'll have to pay for it. B) overclocking can cause unstability or even damage the system, either way it would generate more calls into tech support.