You can try some MPC57xx from freescale, some have like 1,5 MB built-in RAM. But I'd go for a ST32F4xx with external ram, much cheaper. Some of them support QSPI, so if your RAM doesn't have to be super fast it may be an option, if not you can go for 4x512kx8 or 2x256kx16 as external RAM, don't forget that those SRAM are 10 ns or more...
A ST32F439 Discovery board has 8MB SDRAM on board, you may want to have a look at it as a starting platform, it is quite cheap at ~ 20 €,. Schematics are also available.
I'd also love something like that, a small board with CPU+SRAM (I have a ST32F439 Disco and works quite well!) but I don't want a TQFP176 or so and thus I'd go with a PIC32MZ, they have 512k SRAM built-in and QSPI, all in TQFP-64... not bad !. They go for like 10 € each.