Hmm,
You would notice a diffrience, Duron has a newer architecture (Morgan core) + quite strong FPU unit, I would say, the gain in performance would be about 30%......
If you want to keep the sdr, then I would suggest you to get ECS K7S5A mobo. Many people complains about it, but I never had any problems with it, and I ve build about 12 systems based on that board... It has 2 sdr and 2 ddr slots, so it will be possible for you to pick up the ddr stick later.... And it supports virtually all socket A processors, so you in a future should not have any problems upgrading the CPU even further...
Another thing...
this place has 256 Mb of Crucial DDR for below $40.00 shipped(the first one from the top, they call it just micron memory, but it is a real, crucial RAM, I know because I bought like 30 sticks...). My advice is to upgrade the memory as well right now because it will not stay that cheap for ever.....
2 months ago this same stick of Crucial RAM would cost you about $90.00, so the current price is cheap.....
Edit: If you would decide to get the DDR memory, the diffrience in a performance should be even greather
Good luck