512MB. Works just fine.
A little anecdote: yesteday a friend was browsing Crucial.com and made a remark about why some of the PC3200 was so cheap, at ~$48. I told him they were 256MB sticks. He's like, "Wait, my RAM was like $86. Oh, I remember, it's a 512MB stick." I told him that about 2 or 3 years ago, I was able to buy 256MB DDR sticks for $29 at Crucial, and with free shipping too! Yes, they were PC2100 sticks, but they run perfectly fine at PC2700 or even more.
Cliffsnotes: Ram is not really that cheap right now. I'm sticking to the 512MB that I have.