If you just want a replacement motherboard, one can be had for as little as
$29.49. I don't suggest spending much more than that for the sake of an Athlon XP 1700+.
However, you could upgrade your motherboard, CPU and RAM without too much trouble.
motherboard ($64) <--- good overclocking board
CPU ($65)
RAM ($47)
total upgrade price: $176
Then, you could sell your Athlon XP & PC2100 RAM. I bet you'd get at least $30 for each, bringing the net cost of upgrading to ~$115.
So, there you have it. ~$115 for a Sempron 64 2600+ upgrade or $30 to fix your Athlon XP 1700+ setup.