I just wnat to throw this in. Knowing how expensive this suckerz are, I can offer you 4 sticks (128 ea)= 512. Just incase you have any other use for it.
Good luck on 256+. Get the wallet ready for it 
Yeah, I know. I have 4 sticks of 128 that I got off ebay for a pretty good price, actually. And while it's pretty decent for the 1.5ghz P4 I have, I wanted to see if I could boost it.
You can do two, but they have to be matched, and you have to have the dummy cards in the other two slots.
They have to be matched, but I'm not sure if you need the place cards. How much you have will depend what OS you can use. Initially, I got a couple 64mb sticks, and for XP it dragged. Especially for SP2, I'd say minimum 512mb. Win98 would do just fine with 128, maybe even just 64.