Good luck running an OS newer than 95 on a Pentium.

But if you're going to upgrade anyway, toss on Linux, and "cat /proc/cpuinfo". Here's what it says for mine (which has none of the visual clues that robricc mentioned):

processor : 0
vendor_id : GenuineIntel
cpu family : 5
model : 4
model name : Pentium MMX
stepping : 3
cpu MHz : 225.513
fdiv_bug : no
hlt_bug : no
sep_bug : no
f00f_bug : yes
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr mce cx8 mmx
bogomips : 450.56