One thing to remember about most phones is that adding a memory card to it only increases it's storage space and not it's usable memory area. Have you tried downloading a 3-4Mb file and then opening that? I'm thinking that you will be able to open up anything up to about 4Mb before it throws a tantrum again.
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From Nokia
Quote:
Memory Functions
* 10 MB internal dynamic memory for contacts, text messages, multimedia messages, ringing tones, images, video clips, calendar notes, to-do list, and applications
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So it looks like it will only make use of the 10Mb built in for applications etc, your OS takes 4Mb up, the file will require it's 6Mb of ram to be stored in on the phone plus possibly another 6Mb of RAM that it will be using while being uncompressed. (It needs to read the file while writing it's uncompressed format into memory).