Stupid RAM upgrade question

I recently bought a pretty old computer from some friends, It has: Athlon XP 2500+ CPU
Biostar MB
Geforce FX5600
512 MB of DDR 333 RAM

The question is I have an old stick of 128MB DDR 266 laying around somewhere. If I stick that in and the RAM runs at 266 mhz will I have a performance hit or will it perform slightly better due to 640 MB of total ram? Anyway I would assume it would run slightly slower unless the extra ram is really needed to prevent swapping. Ubuntu 9.04 is running just fine on it now, I am just curious.

Thanks

that won't be an upgrade, it will be a downgrade adding a single ddr 266mhz stick because it will force the 333mhz one to work at 266mhz for system compatibility

Eh, that was my guess too. The decrease in the speed of the RAM would be more detrimental to system performance than the tiny added RAM would be beneficial. Thanks for the reply.