first of all, use 2 sticks of the SAME DDR ram, your only running pc2100 speeds even with that 2700 stick in there and still it may not be very efficient.
second, usually with the 2500 bartons, you can just up your FSB in the Bios from 166 staight to 200 without changing anything else and you should run at athlon xp 3200 speeds. At boot up it should also read athlon xp 3200. If you go higher i would start messing slightly with voltage, but running at 3200 is quite a jump and i would stay with that until you get more comfterble with overclocking. But do this at your own risk as i take no responsability in how you overclock.
I had a 2500 barton with a MSI K7N2 Delta L 400 edition nForce 2 chipset with 1 gig of pc 2700 OCZ in dual channel.
I just put my FSB in the bios to 200 and whala....Athlon Xp 3200.
Try it and see how it works, to be honost, it should work out fine.
good luck.
edit here: also always keep an eye on your temps..as they may increase slightly on the cpu.