Update: Check this
link, it is your exact Dell model, got it by using their memory configurer feature. I was right, you only need the PC100 (though PC133 would be compatible, as shown, but it will run at PC100 like the rest of your memory). I'd say the best deal is on the 64MB & 128MB PC100 CAS 2.
Not familiar with the exact model, but if your Dell is a Pentium 450, and is one of their normal home/office 'boxes, then you could go with the PC100 non-ECC. On the other hand, if it's one of Dell's other models, it might require ECC, so check your system manual to find out for sure. If no manual, you ought to be able to find something at Dell's online support pages. The only reason to go PC133 is if you might be upgrading your CPU/motherboard in the next couple/few months, at least you could transfer your newer memory to the new system. Otherwise, save the few bucks by going PC100 ... when you do finally upgrade CPU, etc. in the future, by then you'll be looking at entirely different types of RAM and both PC100 & 133 will be obsolete.