OCZ and Corsair both have DDR500 2GB pairs that are like $150 at newegg right now. I have the Corsair ones, and i had to return them cause they only hit DDR490 and i'm picky lol, but in general i've heard both of them are fantastic brands. I've had good luck with corsair in the past, and i havent used OCZ but i hear they have gotten phenominally better in the last few years.
While running a memory divider won't hurt performance in real world apps too much, running 1:1 will definitely perform better. My 3dmark05 score also dropped by like 1000 when running a divider, although in real world it was much less apparent. The DDR500 modules aren't much more expensive than the DDR400 ones, the only disadvantage is that most of the UCCC DDR500 modules won't hit CAS 2, even at DDR400.