256-400 or 512-266

I can't get more memory right now but I've got an opportunity to switch sticks. I currently run a 256mb samsung ddr-400 but I can grab a 512mb ddr 266. The plus side is I can always switch back later If I want. Which would you run though? All I do is gaming.
Rest of specs.

P4 3.0c
Intel D865GBF board
ATI 9600Pro 128
onboard yadda yadda...

umm dont get the 266, it would be a HUGE bottleneck for your system

So you arent even running dual channel? Are you kidding me?

Right now you are only using 50% of your fsb, if you switch to DDR266, you will only be using 33% of your front side bus. You need to seriously get some dual channel RAM, and it needs to be at least ddr 400, or be able to clock that high.

If your grabbing is free grabbing then there's no harm, but I wouldn't pay $30 or more for a stick of PC-2700, when there's better deals out there.