Sometimes using the search only adds to the confusion; in my case, I see threads for and against each option. Some threads suggest that 4x512 will only work at 166 speeds, others suggest 200 can be had.
Anyway, as per my sig, I've currently got 2x512MB of Kingston HyperX Ultra-Low Latency (2-2-2-5), and it's working dandy. I know Kingston isn't recommended around here, although I never figured out why (been reading/lurking extensively since before I bought the board in late December '05). I do have the timings relaxed, because it wouldn't run stable at the rated speeds, so I'm at the "standard timing" I found in one of the threads here (2-3-3-8, IIRC).
My options are thus:
Add another 2x512 Kingston HyperX, although the non-ULL version (2-3-2-6).
Replace with 2x1GB, either OCZ EL Platinum PC3200 (2-3-2-5) or OCZ Performance PC3200 (3-3-3-8)
The Kingston is TCCD, and I think the OCZ is too, which suits me fine; I don't want to be cranking the voltage too high for OCing anyway.
And I will be doing some overclocking, although I intend to use a divider to keep the RAM near 200 anyway, so I gather that the RAM's overclocking potential isn't terribly important.
Cost of either option (and either OCZ) is essentially the same, since if I go 2x1GB, the replaced 512s will just devolve into another machine that I'd otherwise be buying RAM for as well. As a sidenote, this machine is destined for terabyte+ storage space in the very near future, if that matters.
I'm inclined to go the 2x1GB route, but figured I'd ask here first, get the input of those much wiser than I.
EDIT: For 2x1GB, I could also go OCZ Special Ops or OCZ EL Gold XTC... these guys have more variations of RAM in the same price bracket than I have cousins!