Any Macbook with a 64 bit processor (so since around 2007ish) should run Lion rather well. So any Macbook you buy now will have plenty horsepower to run it, this isnt like Windows where every upgrade necessitates a hardware upgrade.
i.e Snow Leopard was faster than Leopard on the same hardware, with all this iOS influencing and such I highly doubt the system requirements will be more than SL.
Hell even the new Airs should be comfortable running them 😉