I'm building an old-school RAM bank now. Parts needed: 5 * 23LC1024 SPI-RAM, 1*74HC138 3-to-8 Multiplexer (to save Pins on the Arduino).
640 kByte cost less than 20€, including shipping. Each 74HC138 could manage up to 8 RAM modules, so without cascading 1 MByte RAM is possible. It should be possible to switch between the modules in so little time that a continues data stream of 1 MByte/s can be written. By using the 20-€-cents 74HC138 in cascades, even more RAM can be used.
My EEPROMs are useless now, but I hope to have found the solution to my problem 
/* NB: The wiring of the 74HC138 was completely new to me. Pin 8 to GND, Pin 16 to Vcc, Y0-7 to SS/CS on the RAM. A0,1,2 to GPIO on the Arduino for selecting the RAM module. But I think E1,2 need to go to GND and E3 to Vcc so that the chip gets active at all (E1,2,3 are for cascading). /
/ Attached a fritzing sketch. As there is no 23LC1024/SPI I took a 24LC1025/I2C as symbol. I think I will make a staple and bend away the Pins I need directly (CS and HOLD, I think). */
/* The sketches were useless. A friend was so kind to shake a nice building plan out of his CAD program which made real sense. Hundred+ solder points later I finally have a RAM bank. Looks good so far, connecting a LiIon battery to GND/+ doesn't fry the chips. Now only an Arduino has to arrive, finally, so I can test the RAM at least. Could look nicer as I'm really bad in hand work like soldering. */
