
Yeah the XP1600+ (if you can find them esp with the right core) should hit XP2100+ to XP2200+ speeds BUT that is not guaranteed and a lot of SktA mobos can't handle the huge FSB increase needed. You could be very disappointed buying an XP1600+ if you only end up with XP1600+ to XP1800+ speeds considering even the true XP2000+ doesn't cost a whole lot more in the first place.

So when you look at XP o/c'ing you tend to find:
XP1800+ can reach XP2000+ to 2100+
XP1900+ " " XP2100+ to XP2200+
XP2000+ lucky to hit XP2100+
XP2100+ nothing more
XP2200+ nothing more
XP2400+ and up should do well if you can get hold of them

Then you look at actual clock speed and you see that on average you're looking at an increase of 133mhz (9%), wow that's a lot!
RATING SPEED MULTIPLIER
1600+ 1.40ghz 10.5x
1700+ 1.47ghz 11x
1800+ 1.53ghz 11.5x
1900+ 1.60ghz 12x
2000+ 1.67ghz 12.5x
2100+ 1.73ghz 13x
2200+ 1.80ghz 13.5x

Then looking at how P4 tend to o/c we see for CPUs like 1.8@400 and 2.0@400 33% is pretty easy while 40%+ isn't outrageous:
1.8ghz 400FSB should quite easily hit 2.4ghz and 533FSB while 2.6 isn't unheard of.
2.0ghz " " 2.67ghz & 533FSB while 2.8ghz " ".
2.5ghz " " 2.8ghz & 448FSB while 3ghz " ".

Then to some rough prices from pricewatch:
XP1600+ $54
XP1800+ $70
XP2000+ $90
XP2200+ $143
XP2400+ $197
P4 1.8ghz 400FSB $120
P4 2.0ghz 400FSB $145
P4 2.4ghz 533FSB $184
P4 2.5ghz 400FSB $256
P4 2.8ghz 533FSB $462

IMHO the P4's are the clear choice in terms of o/c'ing but as I said the XP still win out because they are both fast and cheap (once the availability of the top XPs comes up their prices will fall).