Price is the major playing factor between P4 and Athlon XP procs. For example, a top of the line P4 runs at a price above $600 (that was for the 2.8 when it hit the market), while the latest-model AthlonXP (the Thoroughbred, I think) started off at around $300 or less.
Performance wise, just go scrounging around a techie website (like Tom's Hardware or SharkyExtreme) and find benchmark data. It pretty much speaks for itself.
Me, I'm starting to be a bit more partial to AMD because they're far less expensive and provide nearly the same performance as their P4 equivalents. Plus they can be overclocked if need be (due to an unlocked clock multiplier, which Intel doesn't have except on engineering samples that never reach the general public).
Oh, and as for memory, I'd say that 256 mb is the bare minimum, and I highly recommend 512 or more. And of course, get DDR if the board supports it. PC2100 is the slowest I'd get, 2700 is more expensive, but probably worth the extra cost. And to save a few bucks (or pounds, euros, franks, marks, etc., depending on nationality), get a 512mb stick instead of 2 256mb sticks. That's a savings of between $20 and $50, depending on brand. Brands I suggest are Apacer, Crucial, and Micron. Generic brands you've never heard of (and a few name-brands with bad track records *cough*PNY*cough*) probably should be avoided.
And up that power supply to 300 or 350 watts first chance you get. Newer processors and mobos, and all those other cards you'll inevitably get, are very power hungry.
And get some case fans to help keep the internals from overheating. In fact, get a chipset cooler for the mobo, and get a processor fan that is rated higher than the processor speed. This'll help ensure that that shiny new piece of plastic won't melt itself after a year of hard use.
Er....sheesh, where'd all that come from?
