i've done this before basically you have to have, for example, with a 15meg file, around 18 if not more on the phone free, as it uses the internal memory as download temp/cache. I did also read somewhere that you sometimes have to have double the downloading file size to download it! But i'm not sure how true that is. I know there are smartphone solutions to this thou, which avoid clearing main memory, such as modaco card cache, but im not sure if this will work on a ppc let alone cache downloads straight to card, but its worth a go. hope this helps.
ps. i've attached modaco card cache just in case you want to try it.