Today, you can get adequately-equipped Socket 754 motherboards starting at about $70. For $120 you can get a Sempron 3100+ plus cooler. Spend another $70 for a 512 MB DDR400 DIMM and your upgrade kit is complete for well under $300, with strong basic performance and good overall overclockability.
The Socket 754 platform is admittedly somewhat controversial at the moment, as the upcoming super-fast (dual core) AMD processors will only be made for Socket 939 or later platforms. It is still possible, however, that processors higher than Athlon 3700+ will be offered for Socket 754.
AMD has shown time and again that it's usually the platforms given up for dead that end up living the longest. The Socket A architecture, which accommodates the Duron, the Athlon XP, and their successor the "Sempron," remains more than sufficient for office systems. Motherboards with NVIDIA's nForce2 chipset or the VIA KT880 are still fine in terms of technology, though they lack the latest innovations such as DDR2 and PCI Express. But since these don't manage to provide that much real-world performance benefit, this isn't as big a disadvantage as it might appear at first.