Tezz: the software is being written and as it is done, the board is being debugged. It for example turned over, that the on-board RTC is inaccessible. The author is fixing such minor flaws now, apart of it the unit I have works very good. I have no idea, when it will be available to get, at what price etc. - these are things the author should be asked about.
Gunstar: the turbo board is an external device, it only plugs into the Atari expansion port and into the motherboard as said above. There is no reason why it should not be compatible with existing RAM upgrades. Basically, as you said, the turbo board has its own memory, and Atari has its own memory (possibly expanded the standard way). These areas remain separate.
There is an operating mode of the board, that sort-of replaces the 64k of Atari memory with the board's memory. This works so that a part of the turbo board memory effectively works as a write-through cache for the first 64k of Atari address space. But, this applies only to *main* 64k of Atari memory. Any expansions (like additional 64k in 130XE banks) won't be cached, so accesses will be slow (still averagely 20% faster than without the board, it has to be said).
ROM upgrades are not a problem either, the boards has its own separate ROM, but normally it boots off the Atari ROM, whatever is in the computer. There are three modes, you can select with a switch in which mode the board should "wake up" when powered up:
"Normal" - this is like the Warp4 board, the memory that resides in the first 64k ("old", i.e. Atari memory, main RAM, ROM, BASIC, RAM banks etc.) is slow (1,77/1,79 MHz), everything else operates at full speed. In this mode the board boots off the Atari ROM.
"Fast" - similarly, the board boots the Atari ROM, but the first 64k memory is cached. This needs special OS (like mine) to operate best, as the OS has to configure the onboard MMU at startup.
"Software" - this is identical to the "Fast" mode, except that the board's internal ROM is booted.
A very nice and powerful device, generally.