For what you're doing, 2GB of RAM is still overkill. 512MB will be fine to start with. If you feel you need more, you can always buy more stuff later. You can also buy DDR of the P4. However, if you want the best performance from it, you'll want to get RDRAM.
I made a sticky with a lot of information regarding everything you wanted and didn't want to know about PC's.
http://forums.zone365.com/showthread.php?s=&threadid=492
Assuming you use 2 hard drives and get a board with on-board RAID, I'll go over the functions.
RAID 0: Striping: this makes the hard drives appear as double the smallest sized hard drive (good reason to buy the same ones). If the data is like this: 123456789; it's written like this: [Disk1:13579], [Disk2:2468].
The main advantage is speed, with no redundancy. If any disk in a RAID 0 array fails, everything is toast.
RAID 1: Mirroring: if you have 2 40GB hard drives, as an example, you will only have 40GB of space with RAID 1.
It writes the data [123456789] to both drives at once: Drive1:123456789], [Drive2:123456789], making it redundant. If one drive goes, you'll only be down a hard drive, not your data.
Personally, I've used a RAID 0 setup and it's not all that in the on-board stuff. It's not worth it, in my opinion. I do look for on-board RAID however, as they can still be used as an extra IDE controller.
There are 2 different companies you'll run into: Promise and Highpoint. I like Highpoint because the controllers are very flexible and you can use them as extra IDE controllers w/o issue.
For the motherboards (finally)
if you're looking into a P4 box, check out the i845 (DDR) boards or the i850 (RDRAM). You cannot go wrong with an Intel chipset.
For Athlon boxes, one of the most stable chipsets is the nForce. On your end of
Abit's TH7-II RAID would be right up you're alley for the P4/RDRAM combo.
The other one to consider would be the Asus P4T-E. Both use the i850 chipset and are widely hailed as the best out their. The Asus does not have RAID however. Abit does and uses a HighPoint chip.