You can do what I did in my iMac G4 and take a 2.5" SATA SSD and a SATA to 40-pin IDE converter. It works quite well under Leopard and shouldn't have any problems with Tiger, but I would ask someone who has actually tried Tiger on an SSD.

SSD I used: http://www.frys.com/product/7395054?site=sr:SEARCH:MAIN_RSLT_PG

2.5" to 3.5" drive holder: http://www.frys.com/product/7001131?site=sr:SEARCH:MAIN_RSLT_PG

SATA to IDE Converter: http://www.amazon.com/exec/obidos/ASIN/B000ZLM9IA

I did try one of those really cheap converters on eBay and it didn't work. Someone on the CubeOwner forums was doing the same thing in their Cube and recommended the Startech adapter.