$70 for a motherboard
and CPU? For that money, you won't get much better than your Duron setup.
This combo is $87 shipped, and features an Athlon XP 2400+. You might be able to find a similar deal on Ebay.
But $70? No, not hardly.
If you can afford it (sell off your Sempron, Duron and motherboard, and you just might), get an
Epox EP-8KDA3I and a
Sempron 64 2800+ for $140 shipped. Then, overclock the bejesus out of the setup. If necessary, you can get a
32-bit 2600+ for $13 less, but I'd go with the 64-bit 2800+ if at all possible (both can be overclocked to 2.4 GHz, but the 2800+ has double the L2 cache).
If you can't afford $127, maybe you should just pick up
this used Athlon 1.3 GHz ($58) to plug into your existing motherboard. You might shave off a few dollars from that price by getting one off Ebay, but I wouldn't count on it.