On the issue of RAM, you may well find that DDR400 (PC3200) is only slightly more expensive than DDR333 (PC2700). Make sure you get your RAM from a good supplier, like Crucial, if you are looking to overclock. As HTRegz mentioned, it won't run at full speed except on an 800Mhz FSB, but on the other hand it will still work. The comments also apply to non Intel chipsets e.g. SiS as used by MSI, amongst others.

It all depends on whether you are considering a hardware upgrade in the future.
If you get a decent mobo (for either AMD or P4) with DDR400 RAM, and a modest CPU, then you can easily double your PC's power by purchasing a new CPU, as you will be able to use your existing mobo & your existing RAM.

When I ordered some DDR333 recently, the supplier was out of stock, and sent me DDR400 instead at no extra charge. I guess this is because the memory chip manafacturers are mainly making DDR400 at the moment.

If you are looking at a gaming PC I would get at least a P4 2.4 or XP 2500, together with a graphics card that fully supports DirectX 9 (preferably with 128Mb), as there will be a host of games released in the next 6 months or so which will only run properly on this type of rig (Doom III, Half life 2 etc. etc.).