If you can get RDRAM for that cheap, then go for it. I haven't checked price on that in a LOOONG time (and I hope I NEVER see that crap again), but back in the day...dang it was expensive. There's no difference between PC800 and PC600 in 99% of the motherboards. It's got slower clock speeds, sure, but you're running an old S423 Pentium 4 so it isn't like you're going to notice 25% slower memory speeds, which would be counteracted by the increase in memory, anyways. On 2000/XP, going from 256Mb to 1Gb can make quite a nice little difference.
Like saint said, though, make sure you've got 4 slots and not 2. Depending on the board, the slots could either take a max of 256Mb or 512Mb - usually 256Mb on the 2 slot boards and 512Mb on the 4 slot boards. It really is a crapshoot, especially if you're using a Dell (which I suspect you are), but most of them, based on my experience, could handle 512Mb chips. I'm guessing you have 2x128Mb now with 2 C-RIMMS filling the two empty slots. You could pick up 2x512Mb and go with 1.284Gb (I think), or pick up 4x256Mb and go with 1Gb and scrap the other ones. Leaving the PC800 in there is fine - it'll simply downclock to the PC600 you add.
Edit...just checked eBay...wow...you can get 4x256Mb for $30 and 2x512Mb for $40ish. Old tech that's actually cheap. Quite surprising.