Just to be sure, your mobo model is just A7N8X, without any suffix (such as A7N8X-X or A7N8X deluxe)?
Yes, Athlon 2400+ native FSB is 266MHz.
Yes, your new mobo can run faster memory, even with your current CPU. Some chipsets (I can think of a VIA chipset) have restrictions on certain combinations of CPU FSB and memory speed (even though supported), but I don't think the nForce2 chipset on your mobo is restricted that way.
But with your current CPU, you won't get much increase in performance using faster memory because the FSB is restricted to 266MHz. Your PC2100 memory (DDR266) provides just the right bandwidth. A faster memory (and a larger bandwidth) will theoretically benefit devices using direct memory access, assuming the system is under full load. But most of the time you won't see any significant difference. Running memory at a frequency different from the FSB also means potentially higher latency.
If your 1GB memory at the moment is in 2 modules, you can leave the memory as it is for the time being, as the nForce2 chipset on the new mobo supports dual channel memory, so that you get increased bandwidth but memory still running at same frequency as FSB. So there's no need to rush off and buy new memory yet.
When you upgrade your CPU, you can buy faster memory (again 2 modules) with the same frequency as the FSB.
Will you notice performance increase with new motherboard? Hope so. The nForce2 chipset is supposed to be the best platform for AMD Athlons (at least up to the Bartons). And Asus is usually a good performer (and uses good quality materials).
Not much difference in ATA-100 and ATA-133, but it's there anyway. XP will have the drivers for ATA-133 disk controller. After installing XP and loading it, insert mobo CD and install mobo drivers.
Not aware your model supports RAID, unless it's the deluxe version which gives RAID support through the SATA controller - but I may be wrong. If you're not using RAID, don't install the Raid drivers if you can help it.
Sound - the onboard sound uses 18-bit sound, Soundblaster Live 5.1 uses 16-bit. How it really translates into a difference in quality I don't know. At this sort of level (compared to say Audigy2 which uses 24-bit), you're probably not very demanding on sound quality, so you may not notice much difference.
The other consideration is performance. Look at the specifications of the mobo and see if it has the SoundStorm APU (audio processing unit). Only the deluxe versions have Soundstorm.
If it does not have the APU, then I would probably use the Soundblaster sound card. This is because without the APU, your computer will use the CPU to process sound, further taxing your CPU.
If it has SoundStorm, then maybe you want to use the onboard sound. But since you have the sound card anyway, try both and see which setup you like.