My Barton 2500+ won't run reliably past ~185Mhz or so either (it's an old one, bought the second week they were first out, but I don't recall the steping info). Above 185 or so I get prime95 errors ... by 200, it causes spontaneous reboots during windows XP loading (even with relaxed memory timings like 8-4-4-3).
I have a Vantec Aeroflow cooler with AS3 on it, running on an Asus A7N8X Deluxe (Rev2.0 BIOS 1007) with 2x512 PC400 Hynix (HY5DU56822BT-D43). I don't think it's my memory either, since this particular one is validated to run PC400 on the later A7N8X-E and -VM variant mobos (and my local store swears it's stable on my board). And it passes memtest fine if I run it async to the CPU at 200Mhz, so I'm pretty sure it's the Barton that's the bottleneck.
As a matter of routine, I run it at 166x12.5 (i.e. "2800+" speed) with slightly agressive mem timings (i.e. 7-3-3-2.5, recommend mem SPD is 8-3-3-3). This configuration is perfectly stable, and quite nice for gaming. If I try to run at around its FSB limit of 185, I can't go above a multiple of 11 without eventual prime95 errors. I also have to reduce the timings to SPD for perfect stability ... frankly not worth the trade-off to have a higher FSB but slightly lower effective CPU speed and slower mem.
I wouldn't normally complain, but it is frustrating to have PC400 mem running at PC333 speeds, especially when so many others seem to be able to get their Bartons to 200Mhz easily. Frankly, I would have been better off with cheaper PC333 running with faster timings ... but it seemed like a good idea to try for the o/c at the time I bought it all (last summer).