The price difference between DDR 266 and 400 is minimal, so i recoment DDR 400. What you could do is buy 512 of DDR 400, and run it with your current 256 of DDR 266. This is similar to what i did. I had a 256mb Stick of DDR 333 underclocked to DDR 266. I added another stick of DDR 400, and underclocked it to DDR 266 and it worked fine. And when i upgrade, i could use my 512mb Stick with the new computer.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.