If you have only have two optical drives, and your HDD's are on the SerialATA channels, then I'd put each CD/DVD drive on it's own channel. That'll maximize the performace between the two (for disc-to-disc copies, for example).
As to the Intel drives, since the 865/875 don't use the IAA, then yeah, I'd install 'em.
Do you have two 512MG sticks or did you mean you have 512MG total? I have a P4C running w/512MG total, and it just drags it's heals in Windows XP. 1GB would help it alot!
Oh, and don't forget to enable HyperThreading - it rocks! You can actually USE the PC when it's doing something like encoding a DVD->DivX, or compressing a DVD w/DVDShrink or Recode2...
Soon to be proud user of Mandrake 10 (again!). SuSE 9 sucks!