Yep - of all on-board resources, RAM is usually the most constrained one.
It's possible to add an SD-card interface, but SD cards and µSD cards are very slow (as seen from an ATmega). Adding a slow device on the SPI bus is inconvenient, because it'll interfere with the RFM12B radio.
Flash memory as included in the JeeLink is a lot faster, but not all models support per-page erase/rewrite, so it may affect how you can use 'em.
RAM is not a practical option - besides, it's volatile. Still, I'd like to create an interface for it one day - if only to re-use all those SDRAM sticks I've swapped out of various notebooks over the years...
The main reason I've not worked on a solution yet, is that I don't know which approach would work well - if any. There is a Memory Plug, which should work fine for website page storage and as small datalogger.
I'm open for suggestions. Meanwhile, I'll keep looking for a way to get rid of that tcp/ip buffer in the EtherCard library. It needs a major rewrite, but since the card itself has 8 Kb, it really is should be possible to get rid of it. The trick is to not generate a complete packet in RAM first.