If mobile Bartons are unavailable where you live, you can achieve your benchmark dream of 2.8 GHz Athlon XP by overclocking an Athlon 64 to just 2.2 GHz with quality RAM and PCI locked Athlon 64 mobo.
2.2 Athlon 64 GHz is way better than 2.2 Athlon XP GHz, it may be roughly equivalent to as much as 2.8-2.9 Athlon XP GHz.