The choice between RDRAM and DDR only applies to P4 systems; Athlon-based systems don't support RDRAM.
On the P4, RDRAM has consistently edged out DDR in performance. But DDR memory controllers are constantly improving, to the point where the DDR333 SiS 645 could challenge the PC800 RDRAM i850, and the new VIA P4X333 and i845G almost match the i850 with PC1066 RDRAM. The trade-off for this extra ~5% RDRAM speed boost is more expensive motherboards and memory. Pricewatch currently lists RDRAM at over twice the cost of DDR. PC1066 RDRAM is also quite scarce, though most current PC800 modules should overclock to that speed.
If you do get a P4, the added cost of RDRAM and a supporting board is probably not worth the miniscule performance difference. Anand called it "intangible", which is true, except when the bill arrives. Pick up a good VIA P4X333 board and pop in some PC2700 DDR. Or wait for the i845G to make it into motherboards.
If you're looking for the highest possible performance, the P4 is on top at the moment with 2.4 GHz chips readily available, while AMD maxes out with the Athlon XP 2100+. Of course, seeking the highest possible performance is invariably foolish because of the innordinate cost incurred. For example, the P4/2.4 costs exactly twice as much as the P4/2.0, yet is hardly 15% faster. Unless you have some urgent life-saving need for that extra 15%, you're better off saving the cash for an even faster CPU down the road. What really matters is value -- bang per buck. In this department, the Athlon remains stronger.
The Athlon XP and the P4 Northwood (the one with 512k cache) are roughly equal at speed grades -- that is, an XP/1600+ is as fast as a P4/1600. Right now, the P4/1600 is the same price as an XP/1900+. Unless you plan to overclock, the Athlon XP is clearly the better choice.
If you do overclock, the decision becomes more complicated. The P4/1600 is all but guaranteed to hit 2000, yet the XP/1900 can also hit that performance level. Many P4/1600's have reached 2400, but an XP/1900 could also reach 2300 (1866 MHz) with some extra effort. One issue to consider is that the P4's multiplier is locked, meaning that overclocking requires raising the FSB to very high levels. This improves performance but can render sensitive peripherals unstable. The Athlon XP can be unlocked to keep the FSB steady. All in all, an overclocking system would be well served by either processor.
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