i have a an XFX readeon HD 5850 Black edition, and i run all my games at 1920x1080 and maxed out with all the AA possible, so far i haven't found a game i cant play at those settings. Crysis warhead gets around 25 fps, 4x AA, 1920x1080 and all maxed out...but its very playable for me. with 8x AA its still playable but there a lots of parts where it drops down to 16 or so and it gets annoying so i leave it at 4x AA
Physx basically adds small details like fabrics tearing and makes small particles like pebbles falling, sparks, water flowing more realistic, its not a HUGE difference though on most of the current games out that have it, but its kinda nice
heres a good comparison with Mirrors Edge:
http://www.youtube.com/watch?v=w0xRJt8rcmY
i tried running the game with physx on my HD 5850 but it starting lagging badly on a certain part where windows were getting shattered, but turns out im not the only one who's getting this prob, and to fix it im getting my friends Nvidia graphics card and just using it as physx card only, so that my 5850 can concentrate on everything else.