1. which mobo? asus K8N (not K8N-E DL) or Gigabyte GA-K8NS (not pro)
Cheap and very nice board is the ABIT NF3-250. It's around $65 and a very nice overclocing board. But between those two, I would choose the Asus.
2. what are the chances of getting 2400mhz with stock volts (since s64 dont have CnQ)?
Get the A64 mobile 3000 754 Clawhammer. OC to around 2.6-2.7. Plus a Clawhammer speed at 2.7 is a Sempron at 3.2. A Duron at 3.4ish.
3. how much of a performance jump from my current system?
Sempron @ 2.6 is like a Duron at 2.8. Sempron are compared to the Intel Celeron and not the P4. An OK jump but could no better with the same budget. Getting the sempron 2600 means you'll need som PC4400 ram to get the speed you want. PC4400 ram cost around $175.
Overclock at Clawhammer to 2.7 and you'll noticable jump. If you want to spend a little more, you can get the 754 3700 Clawhammer, you'll get a free hard drive on newegg. This will also OC to about 2.6 or 2.7 and you would need to get good ram to overclock this 1:1. Just the corsair value will do 225 FSB with timing around 3-4-4-10.
http://www.newegg.com/Product/Product.asp?Item=N82E16819103461
4. or should i get something else?
If you get a low multi CPU, you'll need good ram to run 1:1 and get good overclock. If you get a higher multi, you can buy the cheaper RAM.
I think it's cheaper to get teh 754 3700 Clawhammer (this has the 12 multi) and get the cheap ram. 1 GB of the corsair is around $80.
Basically $205 (processor and HD) +$80 (ram) = $285 -->for 754 3700-Clawhammer, ram, and hard drive
or
$64 processor + $52 hard drive + $175 ram = $291-->Sempron64 2600
The Clawhammer will give you better speed for your buget.