Yup. KT266A was a great mobo and in fact there isn't huge diffs between it and KT333/400 BUT if 333FSB and full CPU support are important it makes sense to get something more capable/flexible (that includes nForce2 of course).
As for the RAM, you definitely do want to get at least DDR333-PC2700 (the fastest official DDR spec), it hardly costs any more than PC2100, which will run happily as PC2100 with ultra fast RAM timings AND still give you a LOT of o/c'ing headroom (esp on KT266A from 266FSB). Brand makes a big diff, mostly to do with longevity, reliability and consistency, Corsair are about tops but any well known RAM should more than do, do check out
http://www.crucial.com
There a 3 key types of XP2100+:
1. Polly marked AX2100D
MT3C which will use 1.75v, run HOT and reach XP2200+ at best.
2. TbredA marked AXDA2100D
KT3C or AXDA2100D
UT3C which will use 1.65/1.60v, run cool BUT only o/c to about XP2300+.
3. TbredB marked AXDA2100D
UT3C which uses 1.60v, runs cool AND should yield XP2800+ or better!
WARNING: For XP2000+ and XP2100+ the marking DUT3C can mean EITHER TbredA or TbredB as both can use 1.60v (that's what the 'U' means). To distinguish you'll either need the stepping code (AIUG
A=TbredA & AIUG
B=TbredB) OR using CPUID which will show 6-8-0=TbredA or 6-8-1=TbredB.
To be sure of getting TbredB you'll either have to cough up the cash for an XP2400+ or else look for an XP1700+ to XP1900+ marked AXDA1700DUT3C which
should always be TbredB as it's the only one to use 1.60v (TbredA uses 1.5v[L] and Polly uses 1.75v[M]). Although AMD don't officially acknowledge the existence of any XP1900+ using TbredB I'd be very surprised if there aren't. No luck in any UK store so far, there are some XP2000+ & XP2100+ which are marked DUT3C but as mentioned these could be TbredA or B!