each IDE interface can handle 2 drives - and most motherboards still come with two IDE interfaces. Mine has 2 IDE and 8 SATA, I think
Onboard networking is quite fine for just about all purposes.
Onboard sound is okay quality, but there can be problems with noise because of all the EMI inside a computer box
- if you hook up digital out to an external amp, that won't be a problem though
I'd stay away from a dual AGP/PCI-e motherboard; don't have any experiences with them, but the idea of two different and power-hungry buses (that are quite different) on one board feels a bit icky.
Btw. as for RAID on on-board controllers - this is
a big can of worms, even if you move to a new board with the same controller. Some of them will not accept that you already have an array if it doesn't know about it (ie., hasn't been configed in that machine), and thus won't "rebuild" the array but will insist on wiping your disks and creating a new array.
As already mentioned, it's probably not worth "upgrading" your old box, so turning it into a "frankenbox" could be smarter. I understand budget is a problem, but if you don't go for the latest & greatest (oh sweet core2duo goodness!), you should still be able to get a decent box without selling your kidneys.
The square 4 pin connector was introduced with pentium4 machines, iirc. Dunno if it's extra juice for the AGP/PCI-e slot or if it's for the CPU - but it's one of those I think. Additionally, modern motherboards have moved from 20- to 24-pin main connectors. I've seen some convertors, which might work, but I wouldn't do that unless the graphics card is a relatively low-end ones. Also note that even some 24-pin PSUs aren't much better than the 20->24 convertor scheme.
As for 32- vs 64-bit XP, I haven't yet seen any reason to install 64bit even though I have an AMD64x2 4400+. It's very little software that can take advantage of the additional address space, and the extra & wider registers don't help that much for much software. So unless you have some software you know
can take advantage of 64bit, it isn't worth it yet imho.