An 1800+ isn't too bad of a CPU. A GeForce 4 Titanium 4200 or Radeon 9600 Pro should have all the CPU power they need with it.
As for the CPU support, the highest end 266 MHz FSB Athlon XP is the 2400+. The real question is whether or not your board can support "Throughbred" core Athlons, versus the older "Palomino" core. The Palomino was available up the 2100+ level, but since then all Athlon XPs from the 2400+ down have transistioned to Tbreds. If your board can support Throughbreds, it will be able to take a 2400+, if not, a 2100+ Palomino would be the limit. (A 2100+ Palomino would be tough to find now, you'd have to do some checking to find one.)