Crucial's 512Mb PC2700 isn't out yet.
I tried Kingmax 256 but had lots of errors from it when I ran Memtest 86. Tried 4 different sticks, had errors on all of then. 2 of the sticks gave me 5000+ errors, the other 2 gave me very few. When I called Kingmax tech support they said these errors meant that it was bad RAM and that I should exchange it. After I got 4 bad sticks I gave up. I think this says something about Kingmax's QC.
Corsair just came out with their 512Mb PC2700 XMS RAM. I got 2 sticks from Mwave ($205/stick). Ran them with Memtest 86 at 340Mhz, CAS2, 2.5volts for 10hrs last night, had zero errors. This stuff looks pretty good so far.
Mushkin should be good too.
It seems hard to go wrong with Crucial, Corsair or Mushkin. A lot of people seem to like the Kingmax but based on the problems I had with it, I'd avoid it for now.