Originally set up my system with 2x 512mb 667mhz corsair XMS2 sticks in dual channel mode.
Went to buy another 2 sticks to upgrade to 2gig but (like an idiot) bought the 1gig 667mhz XMS2 stick instead of the 2x 512 kit.
So I've ordered another 1gig XMS2 stick.
Basically, I will have 2x 1gig and 2x 512meg sticks all the same speed, latencies etc. Soooooo....... what do we think? 2 gig system or 3 gig system? Will the 3 gig system still run in dual channel if all the sizes don't match, or is it OK as long as I keep the channels matched (i.e. 1.5 gig per channel) etc etc etc.
And finally, there will be a week between each stick arriving. So wait and install both at the same time or be impatient and stick one in now and one in next week?
Go ahead and install the first DIMM when it arrives. If the system doesn't like that, pull it out and wait until the 2nd one arrives. 2x512MB + 2x1GB should be fine (just make the channels mirror images of each other as you wrote).