Maybe ChottoED, NascarFool, or one of the other guys can weigh in on the unlocking aspect. I've never done an XP+ cpu. My eyes ant that good
:lol
My 1700+ is not unlocked. The main reason I went with a 1700+ is that you're given a lower default multiplier ( 11x) than with the higher speed XP's. Thus, if you get a core stepping that's conducive to OCing, you merely have to raise the FSB and apply the necessary additional voltage as the FSB ( and net speed of the CPU ) increases. Some of the better steppings are AGOGA, AGOIA, and AROIA. The yields on all the XP+ cpus are better now and that filters down to the lower speed models. When you consider the price difference and lower multi, I think the 1700+ is the sweet spot right now for OCers. Plus, I'm a cheapskate on a budget.
Unless you're planning on unlocking the higher speed models ( 19,20,2100+'s) I don't see the point in dropping that kind of dough on them when you can get similar performance out of a 1700+ all cranked up. A 1700+ @ 1.74 ( 158 FSB) is significantly faster in all benchmarks than a 2100+ running at default speed 13 x 133 FSB.
I've found that XP+ cpu's usually need at least 1.91-1.95v cpu core voltage to really hit their max OCing speeds. In order to get that kind of voltage selection you need to use either the Iwill XP333R, Shuttle AK35GTR, or some of the newer KT333 mobos ( Epox, Shuttle comes to mind ). Most of the KT266A boards max out at 1.85v and that tends to limit your OCing ability, unless of course, you're willing to break out the soldering gun and do physical mods to them. I'm not very adept at that so I'd rather just use mobos that already include the extra core voltage adjustments.
With the Iwill mobo, I've found that if I can get them up over 155 FSB, the memory scores are as good there with the 1/4 PCI divider as they are running at 185 FSB with the 1/5 PCI divider. The ALi MaGik 1C chipset's superior AGP/PCI throughput, in my mind anyway, offsets the slower memory speeds when compared to the KT266A based Shuttle. Ultimately the AK35GTR is slightly faster when running at the same FSB. I think the ALi based XP333R is a more stable OCing platform tho. I've had mine at 185 FSB with an unlocked Duron and it didn't miss a beat. My ram tapped out at that point.
A KT266A mobo like the Shuttle is best suited for 155-160 FSB also in that your mem scores and system speed will really be popping but any higher you're kind of flirting with component damage in that your PCI bus will be above 40 MHz in that there are no 1/5 dividers enabled on any KT266A chipsets. IMHO, Any KT266A that max's out at 1.85v core voltage isn't a very viable OCing solution. Case in point, my "stinky" Abit KR7A-R you see in my sig. It's a nice stable board but all I can get is 1.87v ( set to 1.85v) and it's just not quite enough to kick my 1800+ plus over the 1.7 GHz mark. Which kind of blows when you consider that the same cpu ( 1800+) will do 1.73 on the Iwill.
Predator can prolly tell you more about the KT333 based Epox mobo that he has with regard to running 1/5 dividers and cpu/ram in asynchronous mode. I haven't tried one of those chipsets.
AMD Ph II X4 955 BE 3.2 @ 3.8 GHz | Scythe SCSMZ-2000 | ASRock 880GMH/USB3 | 8 GB G.Skill DDR3 1600 | Radeon HD5670 | Kingston 128GB SSD
Of course a couple WPCREDIT hacks ( registers 61=6C, 78=80 respectively ) are required to get them up around the KT266A speeds. 
rool