The cache is soldered to the board, so yer out of luck there. In theory, the Aladdin 5 could cache up to 512, but the early chipsets had a flaw in the cache tag RAM that caused the 128 MB limitation.

If you bought some slow RAM to save money during the ongoing RAMageddon, you could manually overclock it to achieve greater memory performance. Alternatively, you could use an automatic overclock tool ...