If when you mean upgrade, you mean to simply replace the processor, you wouldn't be able to do that with a P4 because of all the new parts it requires. Unless you have an old Athlon system, and want to upgrade that, definetly a 1.4 GHz Athlon (1.33 is basically same price).
If you're getting a whole new computer, I have just one question; what purpose do you need a computer.
For gaming, currently the P4 at 1.7 GHz + RDRAM is an admirable solution, but considerably more money than an AMD, and you sacrifice a lot of speed in mainstream applications. Upgrading will be expensive on this system too. My advice, if you're a totally hardcore gamer, you should wait a few months for the newer P4s to come out, they will be much faster and better supported.
For all purposes (gaming and mainstream applications) the AMD Athlon at 1.4 GHz with DDR RAM is a great solution. Much cheaper than a 1.7 GHz P4 and much faster in running mainstream applications. Also, although gaming on the P4 is better than the Athlon, you probably won't notice it. Again, you should wait for the newer Athlons to come out. The Athlon 4 with the nForce chipset is coming within the next month.
But if you have to get a computer now, get the 1.4 GHz Athlon, it does everything very well (to be very general).