hmmm... memory prices have been rising fast nowadays. I would say stick with the 2x256 sticks for now, and wait till prices come down. In truth, if the performance of the machine is good enough for you... you should just hold out as long as you can 🙂
Also, there might be some BIOS options where you can run the memory asynchronously from the FSB. ie, ratios that allow you to run the proc at full speed, and the mem @ 133.