1) P5WD2 is an awesome board, but has had RAID problems in the past. I *think * they finally ironed all that out with unofficial BIOS releases (or maybe even stock BIOS with those shipping now), but someone else will need to chime in on that as I am not running RAID and definitely not RAID5.
2) 660 is a very solid proc/good OCer. 4.0GHz (~10% OC) on it should probably be done with stock or lower than stock Vc. I don't know about the Hyper48, but anything better than stock HSF will be a good idea. 600s run cooler than 500s and a LOT cooler than 800s. With that mild of an OC and the little Vc it will take to get it (probably lower than default), any type of aircooling is probably fine.
3) There's a lot of talk floating around about mem at the moment. It depends how much capacity you need and what you want them to do. Cheaper brands like PQI or Patriot or lower lines of name brands like Kingston or Corsair Value Selects might suffice. Again someone else might need to chime in since I've used only Corsair or Mushkin high lines which are generally speed binned to pick the best chips. Value Select versions can be REAL good, but it's a crap shoot.
Probably the best bang for the buck in a "name brand" is the Mushkin XP6400 (~$220 2x512). They can run timings pretty much as tight as Corsair 5400ULs (2x512 for $315) up to about DDR667, but can reach >DDR1000 on default timings and anywhere in between with varying combos of timings between min and defaults.
I don't know what your idea of breaking the bank is, but you'll have top shelf mobo and proc and it would be a shame to stick some no-name DDR2 in there with it
If you really aren't concerned about brand/OCing the mem, just find some DDR2 667 or 675 in the capacity you want with the tightest default timings in a price range you are looking for and then Google reviews for it.