MUST I use TWO Memory Sticks?

This computer (Compaq 5400US) came with two 128M Ram sticks, for a total
of 256m. The computer maximum allowed RAM is 512M. I want to upgrade
to that maximum. There are only 2 slots. I can install TWO 256m
sticks. But can I install just ONE 512m stick?

Finding memory for this machine is a challenge in itself. It needs
168pin - Low density - PC133 SDRAM. Trying to locate using charts
provided by the memory companies all show ONLY up to 256m, and they want
$20 or more per stick. I'm planning to build a more powerful computer,
but this old relic runs, and seems to work well. So, for now it will
do.

But I'm not sticking $40 or more into it. However, I have found on ebay
some used RAM, buy it now, Two 512m sticks of 168pin - Low density -
PC133 SDRAM, for about $10 incl. shipping. Although I can only use one
of them, the price is right, and I'll have a spare.

But I'm not 100% sure if I can use just one 512m sticks, or MUST I use
TWO 256m sticks?