Back in those days 512KB of RAM was common, with simple expansion to 1MB - you probably can't use cheap modern memory, you would need to find memory from that period.
I seem to remember paying £300+ or so for an extra 8MB or so.
It seems a fairly pointless occupation though, if you want 68000 just buy an Amiga off Ebay or similar, otherwise you're going to need to write an OS of some kind for it, and need an EPROM programmer as well. For thta matter an Amiga would probably be the cheaper source of RAM as well.
A 64 pin DIP isn't exactly small. You can buy complete modern embedded MPUs for under $20 with everything included and very small. PIC32MX695L for example.
Yes, usually there's not much use to many MBytes of RAM on a uC, but there are uC's with external memory controllers that you can add RAM modules to. I know a few ARM's have that and I'm assuming there are PIC32's, AVR32's, etc.. that have an external memory bus.
An old 68000 with the full setup is still not going to beat a current 80 or 100Mhz uC. That's not even including the crazy Cortex-A8 ARM's or ARM9's.
If you want to try your hand at video games you might want to take a look at the Parallax Propeller as it can generate composite video. The 68000 was used in a NEO-GEO along with a Z80 back in the 1990. Very little RAM in a video console, it's ROM or CD access that's king.