I have a 8088 IBM PC/XT, in an original 5160 case. I used to have a Tulip card in there which had a selector switch on the back of the unit. Switched up, the system would use the chip on the card, which in this case (from memory) was an 8MHz 80286 card, which effectively doubled the XT's performance (it's clocked at 4.77 MHz).
Flip the switch down, the system would reset and would run from the motherboard's 8088.
I'm looking to either replace the 8088 with a compatible CPU upgrade on the XT, or get a daughtercard like the one talked above as my Tulip card died many moons ago.
If you know of (or have) such a beastie, please let me know if you are willing to sell it.