[WorldIndustries]

Does it make much sense to pay 140 for a Pentium 4 2.2 GHz processor with 512kb of L2 cache, while a Celeron D 3.0 GHz processor with 256 kb L2 is going for 100?

I'm confused on which one to get...

[Jorsoft]

I think that despite the difference in ghz, the P4 is still going to be a much better chip. The L2 is twice the size, for one thing, and that'll give you quite a boost in performance. Besides, the P4 is designed to be better at higher-end apps and will give you better performance.
If you don't need a high performance chip, then yeah get the celeron. They're made for more basic stuff.