Raspberry Pi is actively preventing enthusiasts from upgrading soldered memory to a larger configuration even when the replacement chip is technically compatible. The company says the restriction is intended to discourage unofficial high-capacity resales.
Asus NUC with Intel N150 is what I’ve been looking at. Up-board.org has a credit card sized dev board with the proc, if form factor is a requirement.
There is also the N350 with 8 cores for more power.