I heard there was going to be a 2600+ with a 333Mhz Bus as well, as a lower-end 333 chip. There is another thing that I have noticed, retailers stating incorrect actual clock speeds for 2400+ and 2600+s, stating 2.0Ghz instead of 2.06 for the 2400 and 2.06 instead of 2.13 for the 2600.
I would buy the 333Mhz RAM even if you only get a 266Mhz Bus chip, that way you also have the option of unlocking your processor, lowering the multiplier and running it on a faster bus, for the same speed. (Assuming your board supports this).
This was my original intention with my new rig, I have 2x 2400+ (266Bus) and some 333Mhz, but so far I am still very impressed with my rig and have no need to overclock it yet.
If you get a 333Mhz Bus Chip then I would recommend getting some Decent branded 333 or even some 400+ Mhz RAM, just so that you could run the 400Mhz@333 but at a lower CAS setting, and so boost performance. As for buying 400 RAM just for future proofing, forget it, DDR-II will be out soon for PCs I expect and that will be clocked at 1Ghz+, so 400 will be well obsolete.
Dual Channel DDR is well worth the money from what I've heard if you have a mobo that supports it.