Apple are (I believe) quoting a minimum of 4Gb ram to run Lion. I think in truth it's more like 3Gb, but even so...
I ran it on a 2008 Macbook with Intel Core Duo 2.4Ghz & 4Gb ram, and after a day or two, once the indexing had settled, it ran at a reasonable speed.
Boot up and shutdown times were significantly longer though.
However, I really didn't like the OS (because of the features rather than the performance), and so I've gone back to Snow Leopard.