If you want to estimate the performance gain of your old to your new processor you should take into account what exactly the numbers apply to.
First of all the numbers of the Semprons apply to a similar powerful Intel Celeron, while the Athlons apply to the Pentium 4s.
Furthermore the Athlon 64 CPUs apply to newer versions of Intel CPUs which are more powerful. So an Athlon 64 3200+ is already slightly faster than an Athlon XP 3200+.
Taking all of this into consideration you should have a pretty good performance-gain by upgrading from a socket A Sempron 2800+ to a socket 939 Athlon 3200+.
About the Asrock mainboards...
They are not very good in general. Asrock is the cheapie-brand of Asus and so they are mostly low quality. If you are lucky you won't have any problems with it, but the probability of incompatibilities or other problems with different hardware is way bigger than with a good Asus board. And you are talking about a mainboard with AGP and PCIe and AFAIK these boards have an ULI-chipset. These chipsets are widely known to be crap in terms of performance, stability and compatibility.
An Asus mainboard with an nForce chipset or at least a chipset of VIA or SiS may be a bit more expensive but it pays off in the end. The mainboard is one of the parts in a computer you should always buy with good quality. The mainboard is the part that connects all the different hardware and peripherals of a computer and regulates their communication with each other. A bad layot or cheap parts there can make the whole computer a pure pain in the a**.
And about dual core:
Dual core is the future - no doubt about that. But if you are on a low budget it might still be a bit too early for them. But that depends on what software you want to run on them. Most software still doesn't make any use of the second core. I must admit that I don't know about LFS exactly, but I highly doubt, that it can use the second core. But I can't give you any good suggestions in this part as I don't know what software apart from LFS you are running or are willing to run in the future. And I also can't foresee how quickly the software industry will adapt dual core in the near future. I just wanted to give you a hint on the advantages of dual core and the use of it for your personal needs.
I hope I could help you to make a good decision.