If you're on a budget, you could get a lenovo X200 Tablet, fit a new battery and 8GB of RAM. You get a nice touchscreen which you can orient as you please and 2 minipcie expansion slots.

That is, if you don't require modern OpenGL/gpgpu features, nor plan on doing massively parallel jobs on it (It sports a Core(TM)2 Duo CPU L9400 @ 1.86GHz)

Hypothetically, you _could_ solder a faster processor (not realistically for most of us tho).

Why the X200 and not a more recent one? They changed the keyboard / buttons afterwards, for the worst :(

NB: you might want to patch the bios to remove the mpcie whitelist ibm/lenovo put in. Misewell flash coreboot (requires in-circuit programming, due to flash block write protection)